Tool guide / Dates and time zones

Count days and convert time zones without surprises

Date math looks simple until a month is shorter than expected, a weekend lands in the middle of a deadline, or a clock change removes an hour. Mer Tools calculates these locally in your browser using its built-in calendar and time zone database.

1. Days between two dates

The Date Calculator counts the days from the start date up to the end date. Turn on Include the end date when both days count, such as a booking that covers the 1st through the 7th.

2026-01-01 → 2026-12-25
358 days
11 months, 24 days
51 weeks, 1 day
Weekdays: 256
Weekend days: 102

Weekday counts exclude Saturdays and Sundays. Public holidays differ by country and organization, so subtract them separately when a deadline depends on them.

2. Adding months and years

Adding a month keeps the same day number when it exists and otherwise uses the last day of the shorter month. One month after 2026-01-31 is 2026-02-28 (Saturday). Differences follow the same rule, so adding a result back to the start date returns the end date.

Use the Weekdays unit to find a business-day deadline: one weekday after a Friday is the following Monday. Negative amounts count backward.

3. Meeting times across time zones

The Time Zone Converter reads a local date and time in one zone and shows the same moment elsewhere, including date changes.

America/New_York   2026-10-12 09:00 UTC-04:00
Europe/London      2026-10-12 14:00 UTC+01:00
Asia/Kolkata       2026-10-12 18:30 UTC+05:30
Asia/Tokyo         2026-10-12 22:00 UTC+09:00

Use named zones such as America/New_York instead of abbreviations like EST. Named zones follow daylight saving rules automatically; abbreviations are ambiguous and often stay fixed.

4. Daylight saving gaps and repeats

When clocks spring forward, some local times never happen. A time inside that gap moves forward to the next valid time: 02:30 on 2026-03-08 in New York becomes 03:30 UTC-04:00. When clocks fall back, a repeated time uses its first occurrence.

For logs and APIs, store moments as UTC or ISO 8601 with an offset. The Unix Timestamp Converter converts seconds or milliseconds to readable dates.

Check the source of truth

Time zone rules come from your browser and operating system. Keep them updated, and confirm legally or financially binding dates with the organization that sets them. The same calculators run in the Mer Tools extension. All tools are free; optional donations support development.