Frequently Asked Questions
What is UTC, and is it the same as GMT?
UTC (Coordinated Universal Time) is the modern world time standard, kept by atomic clocks. GMT is the older term for mean solar time at Greenwich. For everyday purposes they show the same time, but UTC is the precise standard that clocks and computers actually follow.
How do time zones work?
The world is divided into regions that share one clock time, each defined as an offset from UTC, ranging from UTC−12:00 to UTC+14:00. Many regions also shift their clocks seasonally for Daylight Saving Time.
Why do some countries have 30- or 45-minute offsets?
Offsets like India’s UTC+05:30, Iran’s UTC+03:30 and Nepal’s UTC+05:45 were chosen so local noon better matches the sun’s position across the country. About a dozen zones use non-whole-hour offsets.
What is Daylight Saving Time and when do clocks change?
DST moves clocks forward one hour in spring and back in autumn to give more evening daylight. The change dates differ by region — North America, Europe and Australia all switch on different weekends, and most of Asia and Africa do not observe DST at all. See our DST tracker for the dates.
What is Unix time?
Unix time is the number of seconds elapsed since 1 January 1970 at 00:00:00 UTC (the Unix epoch). It is the standard way computers store moments in time. Our Unix timestamp converter translates it to readable dates and back.
What is ISO 8601?
ISO 8601 is the international standard for writing dates and times with the largest unit first: 2026-07-03 for a date, or 2026-07-03T14:30:00Z for an exact UTC moment. It sorts naturally and avoids day/month ambiguity.
What makes a year a leap year?
A year is a leap year if it is divisible by 4 — unless it is divisible by 100 and not by 400. So 2024 and 2000 were leap years, while 1900 was not. Leap years add 29 February to keep the calendar aligned with Earth’s orbit.
What is a leap second?
A leap second is an extra second occasionally added to UTC so it stays in step with Earth’s slightly irregular rotation. The international community has agreed to stop inserting leap seconds by 2035.
How do I convert between 12-hour and 24-hour time?
From 1:00 PM onward, add 12 to the hour (1 PM = 13:00, 11 PM = 23:00). Midnight 12 AM is 00:00 and noon 12 PM is 12:00. Our military time converter does this instantly, both ways.
How many time zones are there?
There are 38 different UTC offsets in active use, but the IANA time-zone database defines several hundred named regions, because places sharing an offset can differ in their DST history and rules.
How accurate are the clocks on ddhhmmss?
Our clocks read your device’s own clock and its IANA time-zone database, so they are exactly as accurate as your device — which modern systems keep within a fraction of a second using NTP internet time synchronisation.
Is ddhhmmss free? Do I need an account?
Yes — every tool on ddhhmmss is free, with no account, no sign-up and no ads. The site even works offline once loaded, and you can install it as an app.
Does ddhhmmss collect my data?
No. All tools run entirely in your browser; what you type is never sent to us. Preferences like theme and saved cities are stored only on your device. See our privacy policy for details.
Still curious? Browse the glossary or ask us directly.