Add format_basic method to format as ISO-8601 without field separators

src/main.zig

@@ -206,6 +206,25 @@ pub const DateTime = struct {
         }
         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, @enumToInt(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 "new year" {
@@ -447,3 +466,28 @@ test "custom fmt" {
     try testing.expect(mem.eql(u8, dt_string, "2023-06-10T06:21:22Z"));
     debug.print("Passed\n", .{});
 }
+
+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"));
+    debug.print("Passed\n", .{});
+}

src/timezone.zig

@@ -100,4 +100,27 @@ pub const TimeZone = union(TimeZoneTag) {
             },
         }
     }
+
+    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});
+                    }
+                },
+            },
+        }
+    }
 };