Time Converter

Time looks simple — seconds, minutes, hours — but gets weird at both ends. Project estimation in 'months' or 'weeks' has to decide whether to count weekends. Astronomical time has leap seconds and sidereal days. A 'year' could be 365 days, 365.25, or 365.2425 depending on whether you're ignoring leap years, using the Julian calendar, or the Gregorian. This calculator uses average years (365.25 days) and average months (30.44 days), which is right for spans and wrong for calendar arithmetic.

The second was originally defined as 1/86400 of a mean solar day. It's now defined by atomic physics: 9,192,631,770 oscillations of the cesium-133 atom (1967). This is why atomic clocks are the reference — they measure a physical constant that doesn't drift. The minute (60 seconds) and hour (3,600 seconds) come from Babylonian base-60 arithmetic, inherited through the Greeks and preserved for 4,000 years because 60 has many divisors (easy to split).

The day is 24 hours, but the solar day (noon-to-noon) averages 24 hours and drifts by up to 16 minutes through the year (the equation of time). The sidereal day (reference-to-stars) is 23 hours, 56 minutes, 4 seconds. GPS clocks use atomic time and ignore leap seconds entirely — the two diverge by a second every 18 months or so.

Seconds in a day: 86,400. Seconds in a year (avg): 31,557,600. Hours in a year: 8,766. Minutes in an average month: 43,830. For work time, 'month' usually means 160-176 working hours (20-22 business days × 8 hours).

For human intuitions: a million seconds is 11.6 days. A billion seconds is 31.7 years. A trillion seconds is 31,688 years. The gap between a million and a billion is vast, and the gap between billion and trillion is much vaster — a lesson worth remembering whenever you see dollar amounts.

• Seconds
• Minutes
• Hours
• Days
• Weeks
• Months (avg)
• Years (avg)
• Milliseconds

• Months as 30 days vs actual calendar months. 'Pay in 3 months' is usually calendar-month-based (Jan 15 → Apr 15), not 90 days. Contracts matter — know which.
• Leap year handling. '365 days from Feb 28' in a non-leap year is Feb 28 next year. In a leap year, it's Mar 1 of the year after. Legal contracts define carefully.
• Business days vs calendar days. '5 business days' is ~7 calendar days (one weekend). '30 business days' is ~42 calendar days (6 weekends), close to 6 weeks.
• Daylight saving time. Two DST transitions per year add or remove 1 hour. A meeting 'in 6 months' may happen 1 hour earlier or later than expected.
• GMT vs UTC vs Zulu. GMT is a time zone (UK). UTC is the international reference (atomic, no DST). Zulu is military for UTC. They're the same absolute time — but GMT observes BST in summer, UTC never does. 'GMT+1 in summer' is really UTC+1.

• 1 minute = 60 s.
• 1 hour = 3,600 s.
• 1 day = 86,400 s.
• 1 week = 604,800 s.
• 1 year (365.25 days average) = 31,557,600 s.
• 1 million seconds = 11 days 13 hours 46 min.
• 1 billion seconds ≈ 31 years 251 days.
• 1 trillion seconds ≈ 31,688 years (older than any human civilization).
• Human reaction time: ~250 ms.
• One heartbeat at rest: ~800 ms (75 bpm).
• One blink: ~300 ms.
• Age of the universe: ~4.4 × 10¹⁷ seconds (13.8 billion years).

• For project estimates, plan in 'ideal days' (focused-work equivalent), then multiply by 1.5-2 for interruptions and context switching.
• For deadlines in different time zones, always specify UTC + local. '9 AM ET' is ambiguous during DST transitions.
• For legal billing, keep time in 6-minute increments (0.1 hour). Most billing software rounds to 6 or 15 minutes.

Related: Frequency Converter · Speed Converter · SI Prefix Converter.