Working with Dates: Day Counts, Working Days and Unix Timestamps
Date calculations look simple until month lengths, leap years, weekends and time zones appear. A little care at the start prevents most deadline, billing and scheduling errors, and the rules are easier to remember than they look.
Counting days between two dates
The difference between two calendar dates is the end date minus the start date. From 1 January to 31 January is 30 days, not 31, because the start day is not counted. If you need both days included, add one to the result.
This convention matters for notice periods and billing. A 30-day notice starting on 1 January ends on 31 January, while a period that counts both the first and last day covers 31 calendar days. The Date Difference states its convention in the result so you can see which one you have.
Adding days to a date
Adding days is safer than adding months, because months have 28, 29, 30 or 31 days. Thirty days after 1 January is 31 January, but thirty days after 31 January is 2 March in a non-leap year, since February contributes only 28 days.
- Start 31 January 2026.
- February 2026 has 28 days, so 28 days later is 28 February.
- Two more days reach 2 March 2026.
- The jump in the calendar, not in the arithmetic, is what surprises people.
The Add Days to a Date accepts negative numbers, so it also answers how many days before a date an event fell.
Working days
Business deadlines usually depend on weekdays. The Working Days Calculator counts Monday to Friday and includes both the start and the end date when they fall on weekdays. From Monday 5 October to Friday 16 October 2026 there are 10 working days: five in the first week and five in the second, with the weekend of 10 and 11 October excluded.
Public holidays vary by country and region, so the tool does not subtract them. If a deadline crosses a holiday, remove those days yourself and confirm the rule with the organisation that set it.
Leap years
A year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2024 and 2000 were leap years, while 1900 was not. The extra day is 29 February, which shifts every later date in the year by one.
Calendar-based tools such as the Age Calculator handle the rule automatically. If you calculate by hand, check February before trusting an anniversary date in a leap year.
Countdowns
For events and deadlines, the Days Until a Date compares a target date with today's date on your device. A future date shows the days remaining, and a past date shows how long ago it was, with a weeks-and-days breakdown.
Because the comparison follows your device's local date, two people in different time zones can see different counts for a few hours around midnight. For a shared deadline, agree on one time zone.
Unix timestamps
A Unix timestamp counts seconds since 1 January 1970 UTC. Ten digits usually mean seconds and thirteen mean milliseconds. The value 1,700,000,000 is 14 November 2023 at 22:13:20 UTC.
Mixing the units is the usual mistake: reading a millisecond value as seconds lands tens of thousands of years in the future. Systems that store seconds in a signed 32-bit number run out on 19 January 2038. The Unix Timestamp Converter shows both UTC and local time so you can check the offset.
The same start, different instructions
From 1 January 2026 the calendar, not the arithmetic, decides the result.
| Instruction | Result |
|---|---|
| +30 days | 31 January 2026 |
| +45 days | 15 February 2026 |
| +60 days | 2 March 2026 |
| +90 days | 1 April 2026 |
| +365 days | 1 January 2027 |
2026 is not a leap year, so the +365 row lands on the same date in the next year. In a year that follows a leap year, the same instruction would land one day earlier.
Mistakes that shift deadlines
- Counting the start day twice. The difference between two dates excludes the start; add one only if both ends count.
- Adding months as if they were equal. Use days, or check the resulting date on a calendar.
- Forgetting public holidays. The working-day count covers Monday to Friday only.
- Confusing seconds and milliseconds. Ten digits for seconds, thirteen for milliseconds.
- Ignoring time zones. A countdown changes at local midnight, not at a universal one.
Frequently asked questions
From 1 January to 31 January, how many days is that?
30. The start day is not counted; add one if you need both days included.
What date is 30 days after 31 January 2026?
2 March 2026, because February 2026 has 28 days and the extra two days fall into March.
How many weekdays are there from 5 to 16 October 2026?
10. Both dates are weekdays, and the weekend of 10 and 11 October is excluded.
What is Unix timestamp 1,700,000,000 in UTC?
14 November 2023 at 22:13:20 UTC. It is a ten-digit value, so it counts seconds rather than milliseconds.
- Date Difference – Find the number of days between two dates.
- Working Days Calculator – Count weekdays between two dates.
- Add Days to a Date – Find a date a chosen number of days from a start date.
- Age Calculator – Calculate age from a date of birth.
- Days Until a Date – Count the days to or since a date.
- Unix Timestamp Converter – Convert a Unix timestamp into a readable date.
More guides
- How to Calculate Percentages: 3 Methods and the Mistakes to Avoid
- Understanding Compound Interest: Why Time Matters More Than Amount
- How Loan Payments Work: Principal, Interest and the Cost of Time
- Browse all guides
This guide is for general information and is not financial, tax, legal or medical advice. See our disclaimer.