Date & Time Tools
Date arithmetic, age calculation, timezone conversion, Unix timestamps, countdowns, and more.
Date and time math splits across implementations on month-end handling, leap years, DST, and timezones. The tools in this section answer the common questions immediately — "what date is N days from now?", "how many days between A and B?", "convert this Unix timestamp" — without sending input data anywhere. For business-day and holiday-aware calculations, a dedicated library is still the right choice, as discussed in the linked articles.
Tools in this topic (11)
Date Calculator
Calculate date difference or add/subtract days from a date
Age Calculator
Calculate exact age in years, months, days, and total days from birth date
Time Duration Calculator
Calculate hours and minutes between two times (overnight supported)
Timezone Converter
Convert times between time zones worldwide
Unix Timestamp Converter
Convert Unix timestamps to human-readable dates and vice versa
Day of the Week Calculator
Find the day of the week for any date (also day of year and ISO week)
Japanese Era Year Converter
Convert between Western year and Japanese era year (Reiwa / Heisei / Showa / Taisho / Meiji)
Countdown Timer
Set a countdown timer with hours, minutes, and seconds
Stopwatch
Online stopwatch with lap time recording
Pomodoro Timer
Pomodoro technique timer with work, short break, and long break phases
Batch Epoch Converter
Convert multiple Unix timestamps at once to ISO, UTC, and local time
Related articles
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Age in Japan: full age, kazoe-doshi, and the "day-before-birthday" rule
How age is computed in Japan, the difference between full age and traditional kazoe-doshi counting, the legal rule that age increments on the day before the birthday, and comparisons with other countries.
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Time differences: subtracting times and formatting durations
How to subtract two times, handle day boundaries, ISO 8601 duration notation, and produce human-readable strings.
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Date arithmetic traps: leap years, month ends, business days, time zones
Common pitfalls in adding and subtracting dates, the month-end edge cases, business-day calculations, and time-zone-dependent quirks.
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Timezones, DST, and the IANA tz database: not getting trapped by time
Why "timezone" is more than a UTC offset, what the IANA tz database does, the singular cases at DST transitions, and how to handle time correctly in API design.
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Unix timestamps and why the Year 2038 problem still matters
How Unix time is defined, why the epoch is January 1, 1970, the 32-bit overflow that creates the Y2038 problem, and where it still hides.