Add DateTime.compare for comparisons and ordering

src/main.zig

@@ -238,6 +238,12 @@ pub const WeekDay = enum(u3) {
     wednesday,
 };
 
+pub const Comparison = enum {
+    gt,
+    lt,
+    eq,
+};
+
 pub const DateTime = struct {
     year: Year,
     month: Month,
@@ -301,6 +307,12 @@ pub const DateTime = struct {
         const days = @divTrunc(ts, SECONDS_PER_DAY);
         return @intToEnum(WeekDay, @rem(days, 7));
     }
+
+    pub fn compare(self: Self, other: Self) Comparison {
+        const a = self.toTimestamp();
+        const b = other.toTimestamp();
+        return if (a > b) .gt else if (a < b) .lt else .eq;
+    }
 };
 
 test "get year" {
@@ -380,3 +392,47 @@ test "get weekday 2" {
     };
     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);
+}