About this date difference tool
Two modes: find how many years, months and days separate two dates, or add or subtract any number of days (negative allowed) from a date. The year/month part is calendar-exact: full years first, then full months, with the remainder counted in days; when a month has no matching day, it is clamped to that month's last day (Jan 31 + 1 month = Feb 29 in 2024).
Total days come from the Date.UTC millisecond difference divided by 86400000, so time zones and daylight saving never skew the count across months or years; total weeks include the leftover days. For example, 2024-01-31 to 2024-03-01 is 1 month and 1 day apart, 30 days in total. The add mode also shows the resulting weekday.