A calendar app is wonderful at showing you what's happening when. It's terrible at answering questions like "how many days between two dates?", "what's the date 90 working days from now?", or "what's 9 AM Mumbai time in San Francisco?". These five calculators fill the gap.
The five
1. Age Calculator
Enter a date of birth → get age in years, months, days, hours. Plus next birthday in days. Useful for HR forms, school admissions, and pretending you're younger than you are.
2. Date Difference
Pick two dates → get the difference in days, weeks, months, years. Choose whether to include or exclude the end date (a surprisingly common source of off-by-one bugs).
3. Add / Subtract Days
Pick a date, add (or subtract) days/weeks/months/years, get the resulting date. Handles leap years and month-end edge cases correctly (e.g., Jan 31 + 1 month = Feb 28 or 29).
4. Working Days
The grown-up version of date-diff. Excludes weekends by default. Add your country's holidays (or your company's) for an exact count. Project deadlines, SLA windows, leave planning — all easier.
5. Time Zone Converter
Pick a date, time, and source zone → see it in every other zone you care about. Defaults to a working set (IST, UTC, ET, PT, CET, JST) — add others as needed.
A short note on dates
Dates are deceptively hard. There are leap years (every 4 years, except century years, except every 400). There are time zones (24 of them, plus weird half-hour offsets). There's daylight saving (mostly disappearing, but still active in much of the world). There's the calendar reform of 1582 (October 1582 is short by 10 days in many countries). CalcMaster uses the standard Intl.DateTimeFormat and Intl.RelativeTimeFormat APIs which handle all of this correctly — but you should still sanity-check anything mission-critical.
Open the date & time category for the full list.
