Days between dates, plus date add and subtract
Calendar months vary in length (28 to 31 days), so the calculator works with real calendar arithmetic: a month means the same day-of-month in the following or previous month, and when that day does not exist, such as January 31 plus one month, the date clamps to the last day of the resulting month (February 28 or 29). Day counts span daylight-saving transitions using 24-hour clock days as the world's calendars do.
Counting days between two dates sounds trivial until you hit the details: months of different lengths, leap years, and the question of whether the end date itself counts. This calculator does the arithmetic on real calendar rules. The duration mode reports the span as total days, as weeks with leftover days, as calendar months with days, and as the years-plus-months-plus-days breakdown people use for ages, tenancies, and project timelines. The include-end-date checkbox adds one day, which is what you want for vacation day counts, notice periods, and trial deadlines that expire "on" a given date.
The second mode answers the forward-looking version: given a date, what date lands a number of years, months, weeks, and days away? This is the math behind subscription renewals, warranty periods, pregnancy due dates, and contract deadlines. Years and months are applied first as calendar arithmetic, then weeks and days as simple day steps, and the day is clamped to the end of the month when the target month is shorter, so January 31 plus one month lands on February 28 in a common year and February 29 in a leap year, the convention most legal systems follow.
Weekday names are shown for the computed result, which is often the practical detail: knowing a deadline falls on a weekend lets you plan around it before it bites. For converting individual moments rather than spans, the timestamp converter handles Unix time, and the age calculator specializes the same date math for birthdays.
Set the start and end dates in the duration mode and the total day count appears instantly. By default the span excludes the end date, matching how subtraction works; tick "Include the end date" to add one day, which is the convention for vacation balances, notice periods, and legal deadlines counted inclusively.
Calendar months are 28 to 31 days long, so "3 months and 2 days" is not a fixed number of days; it depends on which months the span crosses. The calculator derives the breakdown from real calendar months while the total-days figure comes from exact 24-hour day counting, so both are correct even when they appear to disagree with naive division.
The result clamps to the last day of the target month: February 28 in a common year, February 29 in a leap year. This is the standard convention because there is no February 31. The same clamping applies to any short target month, for example March 31 plus one month gives April 30.
Yes. Leap years are handled by real calendar arithmetic: February 29 exists when it should, and year totals account for the extra day. Day counts are computed on calendar days, and the add and subtract mode works on local dates so a daylight-saving shift never moves a result off its intended calendar day.
Two counting conventions exist. Exclusive counting, the default, treats a Monday-to-Friday span as 4 days. Inclusive counting adds the end date, giving 5 days, which matches how people count vacation days worked, hotel nights excluding checkout day rules, and statutory response deadlines. Pick the one your situation expects.