Date and Time Tools
Relationship Duration and Anniversary Calculator
Enter a relationship start date to see the calendar duration, elapsed time totals, and the next 20 Valentine dates.
Thailand time calculation
Enter the relationship start date
Uses Asia/Bangkok and assumes a 00:00 start time on the selected date.
Relationship duration
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Quick answer
How long have we been together?
Choose the relationship start date in the Gregorian or Buddhist Era calendar. The tool counts from 00:00 on that date to the current time in Thailand and shows calendar years, months, and days alongside elapsed days, hours, minutes, and seconds.
How to use this tool
- Select the day and month, enter the year, and choose Gregorian or Buddhist Era.
- Select Calculate to see calendar years, months, and days plus elapsed time totals.
- Review the leap-year note and the next 20 Valentine dates.
How the calculation works
Buddhist Era years are converted to Gregorian years by subtracting 543. The calculation uses the proleptic Gregorian calendar and its leap-year rules: years divisible by 4 are leap years except century years not divisible by 400. Calendar duration uses clamped year and month anniversaries, while elapsed totals use the exact millisecond difference in the Asia/Bangkok time zone. Decimal month and year totals are approximations.
Examples
| Example | Input | Expected result |
|---|---|---|
| A 29 February start date | |
1 calendar year, 0 months, and 0 days; 365 elapsed days |
| One day apart | |
0 years, 0 months, and 1 day; 24 elapsed hours |
Limitations
- The input contains a date only and assumes a 00:00 start time in Thailand. A relationship that began at another time can have a different elapsed total.
- Decimal month and year totals use averages of 30.436875 days per month and 365.2425 days per year.
- The tool applies a continuous proleptic Gregorian calendar to every year and does not reproduce each country's historical calendar changes.
- Results change with the current time. Record the date and time checked when using a result in a document.
Frequently asked questions
How is a Buddhist Era year converted to a Gregorian year?
Subtract 543 from the Buddhist Era year. For example, BE 2568 is CE 2025.
How does the tool identify a leap year?
A year divisible by 4 is a leap year unless it is divisible by 100 but not 400. Therefore, 2000 was a leap year and 1900 was not.
Which time zone does the calculation use?
The tool uses Asia/Bangkok, or Thailand time at UTC+7, and treats the start time as 00:00 on the selected date.
Can I enter a future start date?
No. This tool measures elapsed relationship time, so the start date cannot be later than the current time.
References
- ECMAScript Language Specification — Date Objects Verified 2026-07-30
- Microsoft Learn — ThaiBuddhistCalendar Verified 2026-07-30
- Microsoft Learn — DateTime.IsLeapYear Verified 2026-07-30
Content and calculation method last reviewed: