pub const EpochDay = struct {
    day: u47, // u47 = u64 - u17 (because day = sec(u64) / secs_per_day(u17)
    pub fn calculateYearDay(self: EpochDay) YearAndDay {
        var year_day = self.day;
        var year: Year = epoch_year;
        while (true) {
            const year_size = getDaysInYear(year);
            if (year_day < year_size)
                break;
            year_day -= year_size;
            year += 1;
        }
        return .{ .year = year, .day = @as(u9, @intCast(year_day)) };
    }
}