Add `toTimestamp` method, fix function naming to match Zig conventions

src/main.zig

@@ -167,7 +167,7 @@ pub const Offset = union(Sign) {
         }
     }
 
-    fn as_seconds(self: Self) i64 {
+    fn asSeconds(self: Self) i64 {
         return switch (self) {
             .positive => |ofs| blk: {
                 var seconds = @as(i64, ofs.hours) * 3600;
@@ -189,8 +189,8 @@ test "new offsets" {
 }
 
 test "as seconds" {
-    try testing.expectEqual(Offset.new(-4, 30).?.as_seconds(), -16200);
-    try testing.expectEqual(Offset.new(3, null).?.as_seconds(), 10800);
+    try testing.expectEqual(Offset.new(-4, 30).?.asSeconds(), -16200);
+    try testing.expectEqual(Offset.new(3, null).?.asSeconds(), 10800);
 }
 
 pub const TimeZone = union(TimeZoneTag) {
@@ -250,6 +250,39 @@ pub const DateTime = struct {
             .offset => |ofs| ofs,
         };
     }
+
+    pub fn toTimestamp(self: Self) i64 {
+        var seconds = 0;
+        if (self.year.get() < 1970) {
+            var year = Year.new(1970);
+            while (year != self.year) {
+                year = year.previous();
+                seconds -= year.seconds();
+            }
+        } else if (self.year.get() > 1970) {
+            var year = Year.new(1970);
+            while (year != self.year) {
+                seconds += year.seconds();
+                year = year.next();
+            }
+        }
+        var month = Month.january;
+        while (month != self.month) {
+            seconds += month.seconds();
+            month = month.next().?;
+        }
+        seconds += (self.day - 1) * SECONDS_PER_DAY;
+        if (self.hours) |h| {
+            seconds += h * 3600;
+        }
+        if (self.minutes) |m| {
+            seconds += m * 60;
+        }
+        if (self.seconds) |s| {
+            seconds += s;
+        }
+        return seconds;
+    }
 };
 
 test "get year" {