Days Between Dates Calculator

Days from 1 January to the start of each month

Most date gaps people count by hand run from one month to another, and the slow part is adding up the months in between. The middle columns below are that running total: the number of days from 1 January to the 1st of each month, which is what you subtract to get a gap. From 1 March to 1 October in a non leap year is 273 minus 59, so 214 days. In a leap year the same span is 214 days, because the extra 29 February sits before both dates and cancels out. Only spans that straddle a 29 February change length.

The last column is the same figure plus one, and the gap between those two columns is the whole inclusive-versus-exclusive question in one line. 1 September is 243 days after 1 January, and it is also the 244th day of the year, because the day number counts 1 January itself. The calculator above reports the first kind of number.

What common day counts work out as

A raw day count is hard to picture, which is why deadlines get quoted in weeks and months instead. Each row below is the same three conversions the results panel performs on your own dates: whole weeks plus the remainder, the decimal week figure, and the day total divided by the average month length of 30.44 days. The month column is deliberately approximate. There is no fixed number of days in a month, so a 90 day notice period and a 3 month notice period are not the same length, and which is longer depends on which months you land in.

Two rows are worth reading twice. 28 days is exactly 4 weeks with nothing left over, which is why pay cycles and medical courses use it rather than a calendar month. And 365 days comes to 1.00 years rather than a clean 1, because the average Gregorian year is 365.2425 days once the leap cycle is spread across it.

Weekdays and weekend days in each month

A calendar day count is the wrong number for anything scheduled around a working week, and the difference is large: a 30 day month holds either 20, 21 or 22 weekdays depending only on which day it starts on. The columns below count Monday to Friday across whole calendar months, inclusive of the 1st and the last. They do not deduct public holidays, which vary by country and even by nation within the UK, so subtract those yourself. For paid hours inside a single shift the hours calculator handles the clock arithmetic, and the time duration calculator covers gaps shorter than a day.

The full year row is the useful one for anything annual. A 365 day year is 52 whole weeks plus one spare day, so it holds 260 weekdays, or 261 when that spare day is not a Saturday or Sunday. A 366 day year has two spare days and lands on 260, 261 or 262 depending on which day it starts. Deduct your own public holidays from whichever figure applies, then your own leave, to get actual days worked.

Which years are 366 days long

A leap year is any year divisible by four, except century years, which must also be divisible by 400. That is why 2000 was a leap year and 1900 was not, and why the next skipped one is 2100. Any gap that crosses a 29 February is one day longer than the same gap in a neighbouring year, which is the single most common reason a hand count and a calculator disagree. If you are counting from a birth date rather than an arbitrary date, the age calculator reports exact years, months and days instead of a flat day total, and the days until a date calculator counts forward to a deadline.

Counting whole years by multiplying by 365 drifts by about a day every four years. Over a 20 year span that is five days, which matters for interest, tenancies and anything measured in exact days rather than anniversaries.

Inclusive counting, and where the extra day comes from