How Many Days Since the Last Leap Second?
The last leap second was added on 31 December 2016, at the end of the day in Coordinated Universal Time (UTC). It was inserted as 23:59:60 before clocks moved to 00:00:00 on 1 January 2017.
For a practical calendar answer, count from 1 January 2017. The exact number of days since then depends on the date and time used for the calculation, but the calendar difference is straightforward. Leap seconds add a single second, not an extra calendar day.
This distinction matters when comparing civil dates with precise timekeeping. A calendar website can count whole days, while scientific systems may also account for the additional second between UTC and atomic time.
The Date Of The Last Leap Second
The final leap second occurred at 23:59:60 UTC on 31 December 2016. It was announced by the International Earth Rotation and Reference Systems Service (IERS), which monitors how closely Earth rotation matches atomic time.
Leap seconds are usually scheduled at the end of June or December. The 2016 adjustment was added because Earth’s rotation had gradually fallen behind the uniform pace maintained by atomic clocks.
In Australia, the event happened during summer. The corresponding local times were 11:00 am on 1 January 2017 in Australian Eastern Daylight Time, 10:30 am in South Australia, and 8:00 am in Western Australia.
Counting Calendar Days
To count days since the event, use 1 January 2017 as the starting date for ordinary date arithmetic. The leap second occurred immediately before that date began, so most calendar calculations treat the elapsed period as beginning at the start of 2017.
For example, the interval from 1 January 2017 to 1 January 2025 contains 2,922 days. The calculation includes two leap years, 2020 and 2024:
- Six ordinary years contribute 2,190 days
- Two leap years contribute 732 days
- The total is 2,922 calendar days
A date calculator may display a different result if it counts both the starting and ending dates. That is an inclusive count, while elapsed-day calculations normally count the time between two midnight points.
Why Leap Seconds Exist
Earth does not rotate at a perfectly constant speed. Tides, movements in the atmosphere, ocean currents and changes inside the planet can make the length of a solar day vary slightly.
Atomic clocks provide an extremely stable definition of a second. Without occasional adjustments, UTC would slowly drift away from the position of the Sun in the sky. Leap seconds were created to keep civil time broadly aligned with Earth rotation.
A leap second is different from a leap day. A leap day adds 24 hours to February every four years in the usual Gregorian pattern. A leap second adds one second to a UTC minute, producing the unusual timestamp 23:59:60.
UTC And Australian Time Zones
The leap-second announcement is based on UTC, not on local Australian clock time. To interpret it in Australia, apply the relevant time-zone offset and daylight-saving rule.
Sydney, Melbourne, Canberra and Hobart were observing daylight time when the 2016 adjustment occurred. Brisbane remained on Australian Eastern Standard Time, Adelaide used Australian Central Daylight Time, and Perth used Australian Western Standard Time.
For routine planning, Australian calendars and phone clocks do not require special manual changes. Network time services generally handle UTC adjustments behind the scenes, while consumer devices display the local date and time normally.
Leap Seconds Versus Leap Years
Leap years follow a predictable Gregorian rule: a year divisible by four is usually a leap year, except century years that are not divisible by 400. This is why 2000 was a leap year, while 1900 was not, and why 2024 had 366 days.
Leap seconds do not follow a fixed repeating schedule. They depend on observations of Earth rotation and an international decision. Some years have included one, while many years have had none.
For a quick reference covering an unusual alternative calendar question, the Moon day checklist shows why a “day” can have different meanings depending on the system being measured.
Has Another Leap Second Been Added?
No leap second has been inserted since the one at the end of 2016. The absence of a new adjustment means the latest event remains the reference point for current elapsed-time questions.
In 2022, the General Conference on Weights and Measures agreed that UTC should be changed so that the difference between UTC and Earth-rotation time does not exceed a larger limit by or before 2035. This does not immediately abolish leap seconds, but it points towards a future timekeeping system with fewer interruptions.
As a result, a countdown from the 2016 event continues to grow by ordinary calendar days. The total can be checked against any chosen “as of” date rather than treated as one permanently fixed number.
Checking The Number For Any Date
A reliable calculation needs three details: the start date, the end date, and whether the count is exclusive or inclusive. It may also need a time zone if the calculation includes hours, minutes or seconds.
For a date-only answer, enter 1 January 2017 and the target date into a days-between calculator. For an exact UTC duration, use the leap-second moment itself and specify the reference time.
Useful choices depend on the purpose:
- Use calendar days for anniversaries and date planning
- Use elapsed seconds for technical or scientific comparisons
- Use UTC when comparing international timestamps
- Use local Australian time for appointments and public schedules
- Check inclusive counting when matching printed calendars
The following examples show how the same event can be described in different ways:
| Reference point | Practical result |
|---|---|
| Last leap-second date | 31 December 2016 |
| Start of ordinary date count | 1 January 2017 |
| Days to 1 January 2025 | 2,922 days |
| Extra adjustment at the event | 1 second |
| Latest leap second added | 23:59:60 UTC |
A calendar lookup tool such as HowManyDaysIn.com is useful for checking the changing day total, leap-year effects and weekday patterns. For unrelated external reading, a specialist reference site can be kept separate from the calendar calculation itself.
Use 31 December 2016 as the leap-second date, 1 January 2017 as the practical starting point, and your chosen reference date as the endpoint. That method gives a clear calendar answer while preserving the important distinction between whole days and the single extra second added to UTC.