Every result has a clear counting rule.

We use real calendar dates and show the assumptions that matter near each result.

Date difference

The difference excludes the start date and includes the end date. Monday to Tuesday is therefore one calendar day. Negative differences are presented as an absolute distance while the date order remains visible in the inputs.

Business days

A business day is Monday through Friday. Saturday and Sunday are skipped. Public, bank, regional, and company holidays are not removed because those calendars vary by location and organization.

Date addition

Days and weeks move by whole calendar dates. Dates are calculated in UTC so a daylight-saving clock change does not turn a calendar day into a fractional day. A week means seven calendar days, not five working days.

Month-end overflow: check this before planning a deadline

Adding months preserves the day number and carries any excess into the following month. January 31, 2026 plus one month is March 3, 2026, because February has 28 days. It does not clamp the result to February 28. Similarly, February 29, 2024 plus one year becomes March 1, 2025. Use a specific end date if your agreement requires the last day of the month.

Worked examples you can reproduce

• September 4 to September 7, 2026 is three calendar days and one weekday: Monday.
• Three business days after Friday, September 4, 2026 is Wednesday, September 9: Monday is day one, Tuesday day two, and Wednesday day three.
• February 28 to March 1, 2024 is two calendar days because February 29 falls between them.
• The same start and end date returns zero elapsed days. Counting both endpoints would instead describe a one-date inclusive range.

These weekday examples deliberately include no holiday adjustment. If Monday is a holiday in your organization, the three-working-day delivery example would end on Thursday instead. Order cut-off times and the date processing actually starts can move it again; enter the processing start date, not automatically the order date.

Exact age

Exact age is expressed as complete years, remaining complete months, and remaining days between the birth date and the comparison date.

Age uses complete monthly anniversaries and clamps an anniversary to the last day of a shorter month. January 31 to March 1, 2026 is one month and one day; February 29, 2024 to February 28, 2025 is one year. This anniversary convention differs from the overflow rule in the date-addition tool. It does not establish legal age or eligibility rules.

Before using a result

Check the start date, whether the starting day counts, the weekend pattern, holidays, and any month-end rule. DayCountLab provides calendar arithmetic; it does not determine the counting rules in a contract, court notice, or employer policy.

Last updated: September 17, 2026