A year between two dates means one of two things, and mixing them up is where anniversary arithmetic goes wrong. The complete-years figure counts how many anniversaries of the first date have actually passed, which is what service length, warranty cover and qualifying periods use. The decimal figure divides the real day count by the average year length so the span can be plotted or compared.
Between 30 September 1999 and 9 August 2026 exactly 26 anniversaries have passed, with 10 months and 10 days left over. The same span as a decimal is 26.86 years, and the two numbers will never agree except on an exact anniversary. Neither is wrong; they answer different questions.
Anniversaries are found by stepping through the calendar rather than dividing, so a 29 February start date clamps to 28 February in common years instead of drifting into March. That keeps long service dates and policy renewals on the day they were written.
The formula
A- The earlier date
B- The later date
Y- Complete years — anniversaries passed
ΔD- Total elapsed days across the span
How it works, step by step
- Enter the earlier date.
- Enter the later date.
- The gauge shows the span as decimal years.
- The breakdown gives complete years, the next anniversary and the totals.
Worked examples
30 September 1999 to 9 August 2026
26 anniversaries have passed, so the complete-years answer is 26. The decimal figure is 26.86 years because a further 10 months and 10 days have elapsed. The next anniversary falls on Wednesday 30 September 2026.
A leap-day start date
From 29 February 2016 to 28 February 2026 is 10 complete years, because the anniversary clamps back to 28 February in common years. The real day count is 3,652 days.
How to read your score
Frequently asked questions
Why does the decimal not match the whole-year count?
Because the decimal includes the part-year you are partway through. It divides real elapsed days by 365.2425, the average Gregorian year, so it only lines up with the whole count on an exact anniversary.
Which figure should a contract use?
Almost always the complete-years count. Service length, guarantees and qualifying periods are written against anniversaries of a start date, not against a number of days.
How are anniversaries found?
By adding twelve months at a time through the real calendar and clamping to the end of a short month. No division by 365 or 365.25 is involved, so the answer cannot drift.
Is 365.2425 the right average?
It is the mean Gregorian year: 400 years contain 97 leap days, so the average is 365.2425 days. Over four centuries that is 146,097 days in total.
Years between reference
| Later date | Complete years | Decimal years | Total days |
|---|---|---|---|
| 30 September 2004 | 5y 0m 0d | 5.00 | 1,827 |
| 29 September 2009 | 9y 11m 30d | 10.00 | 3,652 |
| 1 October 2014 | 15y 0m 1d | 15.00 | 5,480 |
| 30 September 2019 | 20y 0m 0d | 20.00 | 7,305 |
| 29 February 2024 | 24y 5m 0d | 24.42 | 8,918 |
| 9 August 2026 | 26y 10m 10d | 26.86 | 9,810 |
The 2009 and 2014 rows sit a day either side of the anniversary, which changes the whole-year count.