Counting days between dates
By default the result is the number of days from the start date to the end date, so Monday to Friday of the same week is 4 days. Tick Include the end date to count both days (5), which is what you usually want for leave, hotel nights versus days, or project durations “from … to … inclusive”.
Business days counts Monday to Friday only. Public holidays differ by country and aren't subtracted, so take off any that fall in the period. Months are calendar months: 31 January to 28 February is 28 days, or 0 months and 28 days.
Adding months to a date
Adding a month keeps the day of the month where possible. When the target month is shorter, the result is its last day: 31 January plus one month is 28 February (29 in a leap year), the same as Excel's EDATE.
Excel serial dates
Excel stores dates as numbers: the number of days since 31 December 1899, so 1 January 1900 is 1 and 1 January 2025 is 45658. Times are the fraction of the day, so 45658.5 is noon. When a CSV export or database shows a column of numbers like 45678 where you expected dates, convert them here, or fix a whole column at once with the data cleaner.
Two quirks: Excel treats 1900 as a leap year (serial 60 is the non-existent 29 February 1900), kept for compatibility with Lotus 1-2-3; and old Mac versions of Excel counted from 1904, which shifts every date by 1,462 days. Tick the 1904 option if your dates are four years off. For Unix timestamps, use the timestamp converter.
Frequently asked questions
How do I calculate the number of days between two dates?
Pick the start and end date. The calculator shows the days between them, plus the same span as weeks, months and years and the number of business days.
How do I count business days between two dates?
Enter both dates; the business day count (Monday to Friday) is shown under the total. It matches Excel’s NETWORKDAYS without a holiday list, minus one day unless you tick “Include the end date”.
What date is 90 days from today?
Use “Add or subtract”: today’s date is filled in already, so type 90 and choose days. You can also add business days, weeks, months or years.
How do I convert an Excel serial number to a date?
Type the number into the Serial box. For example, 45678 is 21 January 2025. Decimal parts are converted to a time of day.
What week number is a date?
Enter it in the Excel serial date box; the ISO week number and day of the year are shown along with the serial number.