Add some comments

src/main.zig

@@ -280,6 +280,8 @@ pub const DateTime = struct {
             seconds += month.seconds(self.year);
             month = month.next().?;
         }
+        // The days begin numbering with 1, so on the 5th we have had four full
+        // days plus some remainder. So we take self.days - 1 for our calculation
         seconds += @as(i64, self.day - 1) * SECONDS_PER_DAY;
         if (self.hour) |h| {
             seconds += @as(i64, h - 1) * 3600;