Time calculators

Time arithmetic looks simple until it isn't. A date difference calculation that spans a February 29 is different from one that doesn't. A "30 business days from today" deadline shifts if those 30 days cross a public holiday. A Unix timestamp that's 10 digits long is in seconds; 13 digits means milliseconds. These calculators handle the edge cases correctly so you can trust the result.

The duration subcategory answers "how long" questions about dates and times. The age calculator takes a date of birth and a reference date (defaulting to today) and returns age in years, months, and days — useful for eligibility checks, pediatric reference ranges that shift at 18 months or 5 years, or simply knowing how many days old you are. The date difference calculator works between any two dates and returns the gap in years, months, days, weeks, and total days, with leap years and month-length irregularities handled correctly. A countdown-to-date tool joins them for the case where you need to know how much time remains until a specific moment.

The conversion subcategory covers format and representation shifts. A timezone converter maps a specific date and time from one IANA timezone to another — it handles DST transitions, so "2:00 AM Eastern on the first Sunday of November" resolves correctly rather than returning an ambiguous result. The Unix timestamp converter converts between epoch seconds (or milliseconds) and human-readable UTC dates in both directions. A time-unit converter handles the arithmetic between seconds, minutes, hours, days, weeks, months, and years. An ISO 8601 date parser accepts the various ISO date-time formats and shows what each component means.

The scheduling subcategory is for project and deadline work. A working days calculator counts the business days between two dates, excluding weekends. A business day calculator adds a given number of business days to a start date to find the landing date, and a deadline calculator answers the reverse: given a deadline, how many business days remain? These treat weekends as non-working; public holiday calendars are not yet integrated, so you would adjust the result manually for holidays in your jurisdiction.

The timers subcategory covers active time tracking: a Pomodoro timer for the 25/5 work-break cycle (configurable interval lengths), a general interval timer for HIIT training and similar round/rest patterns, and a stopwatch. These run in the browser tab; they don't sync across devices or integrate with calendar apps.

What these tools are not: a full calendar application, a scheduling system like Calendly, or a time-tracking tool like Toggl. They compute answers to specific time-arithmetic questions; they don't manage events, send reminders, or coordinate across teams. For a shared deadline that multiple people need to act on, a calendar invite or project management tool is the right place for it.