Add new toTimestampNaive method on DateTime so that weekday calculations can be performed for local time rather than for UTC

build.zig

@@ -1,17 +1,26 @@
 const std = @import("std");
 
-pub fn build(b: *std.build.Builder) void {
+pub fn build(b: *std.Build) void {
     // Standard release options allow the person running `zig build` to select
     // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
-    const mode = b.standardReleaseOptions();
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
 
-    const lib = b.addStaticLibrary("chrono", "src/main.zig");
+    const lib = b.addStaticLibrary(.{
-    lib.setBuildMode(mode);
+        .name = "chrono",
-    lib.install();
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+    b.installArtifact(lib);
 
-    const main_tests = b.addTest("src/main.zig");
+    const main_tests = b.addTest(.{
-    main_tests.setBuildMode(mode);
+        .root_source_file = .{ .path = "src/tests.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+    const run_main_tests = b.addRunArtifact(main_tests);
 
     const test_step = b.step("test", "Run library tests");
-    test_step.dependOn(&main_tests.step);
+    test_step.dependOn(&run_main_tests.step);
 }

src/main.zig

@@ -1,245 +1,14 @@
 const std = @import("std");
-const debug = std.debug;
+pub const Year = @import("year.zig").Year;
-const testing = std.testing;
+pub const Month = @import("month.zig").Month;
+pub const tz = @import("timezone.zig");
+pub const Offset = tz.Offset;
+pub const TimeZone = tz.TimeZone;
 
 pub const SECONDS_PER_MINUTE = 60;
 pub const SECONDS_PER_HOUR = 60 * 60;
 pub const SECONDS_PER_DAY = SECONDS_PER_HOUR * 24;
 
-pub const YearTag = enum(u1) {
-    normal,
-    leap,
-
-    fn new(year: i32) YearTag {
-        return if (@rem(year, 4) == 0 and (@rem(year, 100) != 0 or @rem(year, 400) == 0)) .leap else .normal;
-    }
-};
-
-pub const Year = union(YearTag) {
-    normal: i32,
-    leap: i32,
-
-    const Self = @This();
-
-    pub fn new(year: i32) Self {
-        return switch (YearTag.new(year)) {
-            .normal => Self{ .normal = year },
-            .leap => Self{ .leap = year },
-        };
-    }
-
-    pub fn days(self: Self) u16 {
-        return switch (self) {
-            .normal => 365,
-            .leap => 366,
-        };
-    }
-
-    pub fn seconds(self: Self) i64 {
-        return @as(i64, self.days()) * SECONDS_PER_DAY;
-    }
-
-    pub fn get(self: Self) i32 {
-        return switch (self) {
-            .normal => |year| year,
-            .leap => |year| year,
-        };
-    }
-
-    pub fn next(self: Self) Self {
-        return Self.new(self.get() + 1);
-    }
-
-    pub fn previous(self: Self) Self {
-        return Self.new(self.get() - 1);
-    }
-};
-
-test "new year" {
-    try testing.expectEqual(Year.new(2023), Year{ .normal = 2023 });
-    try testing.expectEqual(Year.new(2024), Year{ .leap = 2024 });
-    debug.print("Passed\n", .{});
-}
-
-test "get year" {
-    try testing.expectEqual(Year.new(2023).get(), 2023);
-    try testing.expectEqual(Year.new(2024).get(), 2024);
-    debug.print("Passed\n", .{});
-}
-
-test "next year" {
-    try testing.expectEqual(Year.new(2023).next(), Year{ .leap = 2024 });
-    debug.print("Passed\n", .{});
-}
-
-test "last year" {
-    try testing.expectEqual(Year.new(2024).previous(), Year{ .normal = 2023 });
-    debug.print("Passed\n", .{});
-}
-
-pub const Month = enum(u4) {
-    january = 1,
-    february = 2,
-    march = 3,
-    april = 4,
-    may = 5,
-    june = 6,
-    july = 7,
-    august = 8,
-    september = 9,
-    october = 10,
-    november = 11,
-    december = 12,
-
-    const Self = @This();
-
-    pub fn days(self: Self, year: Year) u5 {
-        return switch (@enumToInt(self)) {
-            1, 3, 5, 7, 8, 10, 12 => 31,
-            2 => switch (year) {
-                .normal => 28,
-                .leap => 29,
-            },
-            else => 30,
-        };
-    }
-
-    pub fn seconds(self: Self, year: Year) u32 {
-        return @as(u32, self.days(year)) * SECONDS_PER_DAY;
-    }
-
-    pub fn next(self: Self) ?Self {
-        const num = @enumToInt(self);
-        return if (num < 12) @intToEnum(Self, num + 1) else null;
-    }
-
-    pub fn previous(self: Self) ?Self {
-        const num = @enumToInt(self);
-        return if (num > 1) @intToEnum(Self, num - 1) else null;
-    }
-};
-
-test "get days in month" {
-    const year = Year.new(2023);
-    const month = Month.february;
-    try testing.expectEqual(month.days(year), 28);
-    debug.print("Passed\n", .{});
-}
-
-test "next month" {
-    try testing.expectEqual(Month.june.next(), .july);
-    try testing.expectEqual(Month.december.next(), null);
-    debug.print("Passed\n", .{});
-}
-
-test "last month" {
-    try testing.expectEqual(Month.june.previous(), .may);
-    try testing.expectEqual(Month.january.previous(), null);
-    debug.print("Passed\n", .{});
-}
-
-pub const TimeZoneTag = enum(u1) {
-    utc,
-    offset,
-};
-
-pub const Sign = enum(u1) {
-    positive,
-    negative,
-};
-
-const HoursMinutes = struct {
-    hours: u4,
-    minutes: ?u6,
-};
-
-pub const Offset = union(Sign) {
-    positive: HoursMinutes,
-    negative: HoursMinutes,
-
-    const Self = @This();
-
-    fn new(hours: i8, minutes: ?u6) ?Self {
-        if (hours > 12 or hours < -12) {
-            return null;
-        }
-        if (minutes) |m| {
-            if (m > 59) return null;
-            if (hours == 0 and m == 0) return null;
-        } else if (hours == 0) return null;
-        if (hours < 0) {
-            const h = @intCast(u4, @as(i8, hours) * -1);
-            return Self{ .negative = .{ .hours = h, .minutes = minutes } };
-        } else {
-            return Self{ .positive = .{ .hours = @intCast(u4, hours), .minutes = minutes } };
-        }
-    }
-
-    fn asSeconds(self: Self) i64 {
-        return switch (self) {
-            .positive => |ofs| blk: {
-                var seconds = @as(i64, ofs.hours) * 3600;
-                if (ofs.minutes) |m| seconds += (@as(i64, m) * 60);
-                break :blk seconds;
-            },
-            .negative => |ofs| blk: {
-                var seconds = @as(i64, ofs.hours) * 3600;
-                if (ofs.minutes) |m| seconds += (@as(i64, m) * 60);
-                break :blk seconds * -1;
-            },
-        };
-    }
-};
-
-test "new offsets" {
-    try testing.expectEqual(Offset.new(-5, null), Offset{ .negative = .{ .hours = 5, .minutes = null } });
-    try testing.expectEqual(Offset.new(3, null), Offset{ .positive = .{ .hours = 3, .minutes = null } });
-    debug.print("Passed\n", .{});
-}
-
-test "as seconds" {
-    try testing.expectEqual(Offset.new(-4, 30).?.asSeconds(), -16200);
-    try testing.expectEqual(Offset.new(3, null).?.asSeconds(), 10800);
-    debug.print("Passed\n", .{});
-}
-
-pub const TimeZone = union(TimeZoneTag) {
-    utc: void,
-    offset: Offset,
-
-    const Self = @This();
-
-    pub fn new(hours: ?i8, minutes: ?u6) ?Self {
-        return if (hours) |h| blk: {
-            if (h == 0) {
-                break :blk .utc;
-            } else if (Offset.new(h, minutes)) |ofs| {
-                break :blk Self{ .offset = ofs };
-            } else {
-                break :blk null;
-            }
-        } else if (minutes) |m| Self{ .offset = Offset.new(0, m).? } else .utc;
-    }
-};
-
-test "new timezone" {
-    const tz = TimeZone.new(-5, null).?;
-    try testing.expectEqual(@as(TimeZoneTag, tz), .offset);
-    switch (tz) {
-        .offset => |ofs| try testing.expectEqual(ofs, Offset{ .negative = .{ .hours = 5, .minutes = null } }),
-        else => unreachable,
-    }
-    debug.print("Passed\n", .{});
-}
-
-test "new timezone utc" {
-    const tz0 = TimeZone.new(null, null).?;
-    const tz1 = TimeZone.new(0, null).?;
-    try testing.expectEqual(@as(TimeZoneTag, tz0), .utc);
-    try testing.expectEqual(@as(TimeZoneTag, tz1), .utc);
-    debug.print("Passed\n", .{});
-}
-
 pub const WeekDay = enum(u3) {
     thursday = 0,
     friday,
@@ -278,7 +47,7 @@ pub const DateTime = struct {
         };
     }
 
-    pub fn toTimestamp(self: Self) i64 {
+    fn toTimestampNaive(self: Self) i64 {
         var seconds: i64 = 0;
         if (self.year.get() < 1970) {
             var year = Year.new(1970);
@@ -310,6 +79,11 @@ pub const DateTime = struct {
         if (self.second) |s| {
             seconds += s;
         }
+        return seconds;
+    }
+
+    pub fn toTimestamp(self: Self) i64 {
+        var seconds = self.toTimestampNaive();
         if (self.getOffset()) |ofs| seconds -= ofs.asSeconds();
         return seconds;
     }
@@ -345,7 +119,7 @@ pub const DateTime = struct {
                 seconds -= 60;
                 minutes -= 1;
             }
-            const second = @intCast(u6, seconds + 60);
+            const second = @as(u6, @intCast(seconds + 60));
             return Self{
                 .year = year,
                 .month = month.?,
@@ -376,15 +150,15 @@ pub const DateTime = struct {
             return Self{
                 .year = year,
                 .month = month,
-                .day = @intCast(u8, day),
+                .day = @as(u8, @intCast(day)),
-                .hour = @intCast(u5, hour),
+                .hour = @as(u5, @intCast(hour)),
-                .minute = @intCast(u6, minute),
+                .minute = @as(u6, @intCast(minute)),
-                .second = @intCast(u6, seconds),
+                .second = @as(u6, @intCast(seconds)),
                 .tz = .utc,
             };
         } else {
             return Self{
-                .year = Year.new(0),
+                .year = Year.new(1970),
                 .month = .january,
                 .day = 1,
                 .hour = 0,
@@ -400,9 +174,9 @@ pub const DateTime = struct {
     }
 
     pub fn weekday(self: Self) WeekDay {
-        const ts = self.toTimestamp();
+        const ts = self.toTimestampNaive();
         const days = @divTrunc(ts, SECONDS_PER_DAY);
-        return @intToEnum(WeekDay, @rem(days, 7));
+        return @as(WeekDay, @enumFromInt(@rem(days, 7)));
     }
 
     pub fn compare(self: Self, other: Self) Comparison {
@@ -410,153 +184,47 @@ pub const DateTime = struct {
         const b = other.toTimestamp();
         return if (a > b) .gt else if (a < b) .lt else .eq;
     }
+
+    pub fn format(
+        self: Self,
+        comptime fmt: []const u8,
+        options: std.fmt.FormatOptions,
+        writer: anytype,
+    ) !void {
+        _ = fmt;
+        _ = options;
+
+        try writer.print("{s}-{d:0>2}-{d:0>2}", .{
+            self.year, @intFromEnum(self.month), self.day,
+        });
+        if (self.hour) |h| {
+            try writer.print("T{d:0>2}", .{h});
+            if (self.minute) |m| {
+                try writer.print(":{d:0>2}", .{m});
+                if (self.second) |s| {
+                    try writer.print(":{d:0>2}", .{s});
+                }
+            }
+        }
+        try writer.print("{s}", .{self.tz});
+    }
+
+    pub fn format_basic(
+        self: Self,
+        writer: anytype,
+    ) !void {
+        try writer.print("{s}{d:0>2}{d:0>2}", .{
+            self.year, @intFromEnum(self.month), self.day,
+        });
+        if (self.hour) |h| {
+            try writer.print("T{d:0>2}", .{h});
+            if (self.minute) |m| {
+                try writer.print("{d:0>2}", .{m});
+                if (self.second) |s| {
+                    try writer.print("{d:0>2}", .{s});
+                }
+            }
+        }
+        try self.tz.format_basic(writer);
+    }
 };
-
-test "get year" {
-    const dt = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 12,
-        .hour = 1,
-        .minute = 5,
-        .second = 14,
-        .tz = TimeZone.new(-5, null).?,
-    };
-    try testing.expectEqual(dt.getYear(), 2023);
-    debug.print("Passed\n", .{});
-}
-
-test "get offset" {
-    const dt = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 12,
-        .hour = 1,
-        .minute = 5,
-        .second = 14,
-        .tz = TimeZone.new(-5, null).?,
-    };
-    try testing.expectEqual(dt.getOffset().?, Offset{ .negative = .{ .hours = 5, .minutes = null } });
-    debug.print("Passed\n", .{});
-}
-
-test "to timestamp utc" {
-    const dt = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 13,
-        .hour = 5,
-        .minute = 21,
-        .second = 22,
-        .tz = .utc,
-    };
-    try testing.expectEqual(dt.toTimestamp(), 1686633682);
-    debug.print("Passed\n", .{});
-}
-
-test "to timestamp negative offset" {
-    const dt = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 13,
-        .hour = 0,
-        .minute = 21,
-        .second = 22,
-        .tz = TimeZone.new(-5, null).?,
-    };
-    try testing.expectEqual(dt.toTimestamp(), 1686633682);
-    debug.print("Passed\n", .{});
-}
-
-test "conversions" {
-    const ts = std.time.timestamp();
-    const dt = DateTime.fromTimestamp(ts);
-    try testing.expectEqual(dt.toTimestamp(), ts);
-    debug.print("Passed\n", .{});
-}
-
-test "now" {
-    const ts = std.time.timestamp();
-    const dt = DateTime.now();
-    try testing.expect(ts <= dt.toTimestamp());
-    // Make sure they're identical at least to the tens place, since it's possible
-    // for them to vary by a second or more depending on system resources
-    const a = @divTrunc(ts, 10);
-    const b = @divTrunc(dt.toTimestamp(), 10);
-    try testing.expectEqual(a, b);
-    debug.print("Passed\n", .{});
-}
-
-test "get weekday" {
-    const dt = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 13,
-        .hour = 6,
-        .minute = 21,
-        .second = 22,
-        .tz = .utc,
-    };
-    try testing.expectEqual(dt.weekday(), .tuesday);
-    debug.print("Passed\n", .{});
-}
-
-test "get weekday 2" {
-    const dt = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 10,
-        .hour = 6,
-        .minute = 21,
-        .second = 22,
-        .tz = .utc,
-    };
-    try testing.expectEqual(dt.weekday(), .saturday);
-    debug.print("Passed\n", .{});
-}
-
-test "ordering lt" {
-    const a = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 10,
-        .hour = 6,
-        .minute = 21,
-        .second = 22,
-        .tz = .utc,
-    };
-    const b = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 10,
-        .hour = 6,
-        .minute = 21,
-        .second = 23,
-        .tz = .utc,
-    };
-    try testing.expectEqual(a.compare(b), .lt);
-    debug.print("Passed\n", .{});
-}
-
-test "ordering gt" {
-    const a = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 10,
-        .hour = 6,
-        .minute = 21,
-        .second = 22,
-        .tz = .utc,
-    };
-    const b = DateTime{
-        .year = Year.new(2023),
-        .month = .june,
-        .day = 10,
-        .hour = 6,
-        .minute = 21,
-        .second = 22,
-        .tz = TimeZone.new(1, null).?,
-    };
-    try testing.expectEqual(a.compare(b), .gt);
-    debug.print("Passed\n", .{});
-}

src/month.zig

@@ -0,0 +1,47 @@
+const std = @import("std");
+const debug = std.debug;
+const testing = std.testing;
+const Year = @import("year.zig").Year;
+const SECONDS_PER_DAY = @import("main.zig").SECONDS_PER_DAY;
+
+pub const Month = enum(u4) {
+    january = 1,
+    february = 2,
+    march = 3,
+    april = 4,
+    may = 5,
+    june = 6,
+    july = 7,
+    august = 8,
+    september = 9,
+    october = 10,
+    november = 11,
+    december = 12,
+
+    const Self = @This();
+
+    pub fn days(self: Self, year: Year) u5 {
+        return switch (@intFromEnum(self)) {
+            1, 3, 5, 7, 8, 10, 12 => 31,
+            2 => switch (year) {
+                .normal => 28,
+                .leap => 29,
+            },
+            else => 30,
+        };
+    }
+
+    pub fn seconds(self: Self, year: Year) u32 {
+        return @as(u32, self.days(year)) * SECONDS_PER_DAY;
+    }
+
+    pub fn next(self: Self) ?Self {
+        const num = @intFromEnum(self);
+        return if (num < 12) @enumFromInt(num + 1) else null;
+    }
+
+    pub fn previous(self: Self) ?Self {
+        const num = @intFromEnum(self);
+        return if (num > 1) @enumFromInt(num - 1) else null;
+    }
+};

src/tests.zig

@@ -0,0 +1,252 @@
+const std = @import("std");
+const debug = std.debug;
+const mem = std.mem;
+const testing = std.testing;
+const DateTime = @import("main.zig").DateTime;
+const Year = @import("year.zig").Year;
+const Month = @import("month.zig").Month;
+const tz = @import("timezone.zig");
+const Offset = tz.Offset;
+const TimeZone = tz.TimeZone;
+
+test "new year" {
+    try testing.expectEqual(Year.new(2023), Year{ .normal = 2023 });
+    try testing.expectEqual(Year.new(2024), Year{ .leap = 2024 });
+}
+
+test "get year" {
+    try testing.expectEqual(Year.new(2023).get(), 2023);
+    try testing.expectEqual(Year.new(2024).get(), 2024);
+}
+
+test "next year" {
+    try testing.expectEqual(Year.new(2023).next(), Year{ .leap = 2024 });
+}
+
+test "last year" {
+    try testing.expectEqual(Year.new(2024).previous(), Year{ .normal = 2023 });
+}
+
+test "get days in month" {
+    const year = Year.new(2023);
+    const month = Month.february;
+    try testing.expectEqual(month.days(year), 28);
+}
+
+test "next month" {
+    try testing.expectEqual(Month.june.next(), .july);
+    try testing.expectEqual(Month.december.next(), null);
+}
+
+test "last month" {
+    try testing.expectEqual(Month.june.previous(), .may);
+    try testing.expectEqual(Month.january.previous(), null);
+}
+
+test "new offsets" {
+    try testing.expectEqual(Offset.new(-5, null), Offset{ .negative = .{ .hours = 5, .minutes = null } });
+    try testing.expectEqual(Offset.new(3, null), Offset{ .positive = .{ .hours = 3, .minutes = null } });
+}
+
+test "as seconds" {
+    try testing.expectEqual(Offset.new(-4, 30).?.asSeconds(), -16200);
+    try testing.expectEqual(Offset.new(3, null).?.asSeconds(), 10800);
+}
+
+test "new timezone" {
+    const zone = TimeZone.new(-5, null).?;
+    try testing.expectEqual(@as(tz.TimeZoneTag, zone), .offset);
+    switch (zone) {
+        .offset => |ofs| try testing.expectEqual(ofs, Offset{ .negative = .{ .hours = 5, .minutes = null } }),
+        else => unreachable,
+    }
+}
+
+test "new timezone utc" {
+    const tz0 = TimeZone.new(null, null).?;
+    const tz1 = TimeZone.new(0, null).?;
+    try testing.expectEqual(@as(tz.TimeZoneTag, tz0), .utc);
+    try testing.expectEqual(@as(tz.TimeZoneTag, tz1), .utc);
+}
+
+test "get year from DateTime" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 12,
+        .hour = 1,
+        .minute = 5,
+        .second = 14,
+        .tz = TimeZone.new(-5, null).?,
+    };
+    try testing.expectEqual(dt.getYear(), 2023);
+}
+
+test "get offset" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 12,
+        .hour = 1,
+        .minute = 5,
+        .second = 14,
+        .tz = TimeZone.new(-5, null).?,
+    };
+    try testing.expectEqual(dt.getOffset().?, Offset{ .negative = .{ .hours = 5, .minutes = null } });
+}
+
+test "to timestamp utc" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 13,
+        .hour = 5,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    try testing.expectEqual(dt.toTimestamp(), 1686633682);
+}
+
+test "to timestamp negative offset" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 13,
+        .hour = 0,
+        .minute = 21,
+        .second = 22,
+        .tz = TimeZone.new(-5, null).?,
+    };
+    try testing.expectEqual(dt.toTimestamp(), 1686633682);
+}
+
+test "conversions" {
+    const ts = std.time.timestamp();
+    const dt = DateTime.fromTimestamp(ts);
+    try testing.expectEqual(dt.toTimestamp(), ts);
+}
+
+test "now" {
+    const ts = std.time.timestamp();
+    const dt = DateTime.now();
+    try testing.expect(ts <= dt.toTimestamp());
+    // Make sure they're identical at least to the tens place, since it's possible
+    // for them to vary by a second or more depending on system resources
+    const a = @divTrunc(ts, 10);
+    const b = @divTrunc(dt.toTimestamp(), 10);
+    try testing.expectEqual(a, b);
+}
+
+test "get weekday" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 13,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    try testing.expectEqual(dt.weekday(), .tuesday);
+}
+
+test "get weekday 2" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    try testing.expectEqual(dt.weekday(), .saturday);
+}
+
+test "ordering lt" {
+    const a = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    const b = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 23,
+        .tz = .utc,
+    };
+    try testing.expectEqual(a.compare(b), .lt);
+}
+
+test "ordering gt" {
+    const a = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    const b = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = TimeZone.new(1, null).?,
+    };
+    try testing.expectEqual(a.compare(b), .gt);
+}
+
+test "custom fmt" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = .utc,
+    };
+    const dt_string = try std.fmt.allocPrint(
+        testing.allocator,
+        "{s}",
+        .{dt},
+    );
+    defer testing.allocator.free(dt_string);
+    try testing.expect(mem.eql(u8, dt_string, "2023-06-10T06:21:22Z"));
+}
+
+test "fmt basic" {
+    const dt = DateTime{
+        .year = Year.new(2023),
+        .month = .june,
+        .day = 10,
+        .hour = 6,
+        .minute = 21,
+        .second = 22,
+        .tz = TimeZone.new(-4, null).?,
+    };
+    var dt_array = std.ArrayList(u8).init(testing.allocator);
+    defer dt_array.deinit();
+    var writer = dt_array.writer();
+    try dt.format_basic(writer);
+    try testing.expect(mem.eql(u8, dt_array.items, "20230610T062122-04"));
+    const dt_string = try std.fmt.allocPrint(
+        testing.allocator,
+        "{s}",
+        .{dt},
+    );
+    defer testing.allocator.free(dt_string);
+    try testing.expect(mem.eql(u8, dt_string, "2023-06-10T06:21:22-04"));
+}

src/timezone.zig

@@ -0,0 +1,126 @@
+const std = @import("std");
+const debug = std.debug;
+const testing = std.testing;
+
+pub const TimeZoneTag = enum(u1) {
+    utc,
+    offset,
+};
+
+pub const Sign = enum(u1) {
+    positive,
+    negative,
+};
+
+const HoursMinutes = struct {
+    hours: u4,
+    minutes: ?u6,
+};
+
+pub const Offset = union(Sign) {
+    positive: HoursMinutes,
+    negative: HoursMinutes,
+
+    const Self = @This();
+
+    pub fn new(hours: i8, minutes: ?u6) ?Self {
+        if (hours > 12 or hours < -12) {
+            return null;
+        }
+        if (minutes) |m| {
+            if (m > 59) return null;
+            if (hours == 0 and m == 0) return null;
+        } else if (hours == 0) return null;
+        if (hours < 0) {
+            const h: u4 = @intCast(@as(i8, hours) * -1);
+            return Self{ .negative = .{ .hours = h, .minutes = minutes } };
+        } else {
+            return Self{ .positive = .{ .hours = @intCast(hours), .minutes = minutes } };
+        }
+    }
+
+    pub fn asSeconds(self: Self) i64 {
+        return switch (self) {
+            .positive => |ofs| blk: {
+                var seconds = @as(i64, ofs.hours) * 3600;
+                if (ofs.minutes) |m| seconds += (@as(i64, m) * 60);
+                break :blk seconds;
+            },
+            .negative => |ofs| blk: {
+                var seconds = @as(i64, ofs.hours) * 3600;
+                if (ofs.minutes) |m| seconds += (@as(i64, m) * 60);
+                break :blk seconds * -1;
+            },
+        };
+    }
+};
+
+pub const TimeZone = union(TimeZoneTag) {
+    utc: void,
+    offset: Offset,
+
+    const Self = @This();
+
+    pub fn new(hours: ?i8, minutes: ?u6) ?Self {
+        return if (hours) |h| blk: {
+            if (h == 0) {
+                break :blk .utc;
+            } else if (Offset.new(h, minutes)) |ofs| {
+                break :blk Self{ .offset = ofs };
+            } else {
+                break :blk null;
+            }
+        } else if (minutes) |m| Self{ .offset = Offset.new(0, m).? } else .utc;
+    }
+
+    pub fn format(
+        self: Self,
+        comptime fmt: []const u8,
+        options: std.fmt.FormatOptions,
+        writer: anytype,
+    ) !void {
+        _ = fmt;
+        _ = options;
+
+        switch (self) {
+            .utc => try writer.writeAll("Z"),
+            .offset => |ofs| switch (ofs) {
+                .positive => |p| {
+                    try writer.print("+{d:0>2}", .{p.hours});
+                    if (p.minutes) |m| {
+                        try writer.print(":{d:0>2}", .{m});
+                    }
+                },
+                .negative => |n| {
+                    try writer.print("-{d:0>2}", .{n.hours});
+                    if (n.minutes) |m| {
+                        try writer.print(":{d:0>2}", .{m});
+                    }
+                },
+            },
+        }
+    }
+
+    pub fn format_basic(
+        self: Self,
+        writer: anytype,
+    ) !void {
+        switch (self) {
+            .utc => try writer.writeAll("Z"),
+            .offset => |ofs| switch (ofs) {
+                .positive => |p| {
+                    try writer.print("+{d:0>2}", .{p.hours});
+                    if (p.minutes) |m| {
+                        try writer.print("{d:0>2}", .{m});
+                    }
+                },
+                .negative => |n| {
+                    try writer.print("-{d:0>2}", .{n.hours});
+                    if (n.minutes) |m| {
+                        try writer.print("{d:0>2}", .{m});
+                    }
+                },
+            },
+        }
+    }
+};

src/year.zig

@@ -0,0 +1,70 @@
+const std = @import("std");
+const debug = std.debug;
+const testing = std.testing;
+const SECONDS_PER_DAY = @import("main.zig").SECONDS_PER_DAY;
+
+pub const YearTag = enum(u1) {
+    normal,
+    leap,
+
+    fn new(year: i32) YearTag {
+        return if (@rem(year, 4) == 0 and (@rem(year, 100) != 0 or @rem(year, 400) == 0)) .leap else .normal;
+    }
+};
+
+pub const Year = union(YearTag) {
+    normal: i32,
+    leap: i32,
+
+    const Self = @This();
+
+    pub fn new(year: i32) Self {
+        return switch (YearTag.new(year)) {
+            .normal => Self{ .normal = year },
+            .leap => Self{ .leap = year },
+        };
+    }
+
+    pub fn days(self: Self) u16 {
+        return switch (self) {
+            .normal => 365,
+            .leap => 366,
+        };
+    }
+
+    pub fn seconds(self: Self) i64 {
+        return @as(i64, self.days()) * SECONDS_PER_DAY;
+    }
+
+    pub fn get(self: Self) i32 {
+        return switch (self) {
+            .normal => |year| year,
+            .leap => |year| year,
+        };
+    }
+
+    pub fn next(self: Self) Self {
+        return Self.new(self.get() + 1);
+    }
+
+    pub fn previous(self: Self) Self {
+        return Self.new(self.get() - 1);
+    }
+
+    pub fn format(
+        self: Self,
+        comptime fmt: []const u8,
+        options: std.fmt.FormatOptions,
+        writer: anytype,
+    ) !void {
+        _ = fmt;
+        _ = options;
+
+        const year = self.get();
+        if (year > 0) {
+            try writer.print("{d:0>4}", .{@as(u32, @intCast(year))});
+        } else {
+            try writer.print("{d:0>4}", .{year});
+        }
+    }
+};