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

src/main.zig

@@ -47,7 +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);
@@ -79,6 +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;
     }
@@ -169,7 +174,7 @@ 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 @as(WeekDay, @enumFromInt(@rem(days, 7)));
     }