What this calculator works out
This calculator takes two date and time values and returns the interval between them in days, hours and minutes, plus a total in decimal hours.
Both forms are useful. Days and hours is how an interval is described; decimal hours is what feeds into thermal accumulation, elimination and cooling calculations. This is a teaching and planning aid, not a casework tool.
Date and time
Before you calculate
- Both values use your device's local time zone. Records from different sources may not.
- The end must be after the start; the calculator rejects a negative interval rather than showing one.
- Watch for daylight saving transitions. An interval spanning a clock change is an hour longer or shorter in local time than the elapsed time suggests.
- The decimal hours figure is the one to carry into other calculations.
Thermal accumulation, drug elimination and body cooling all need a single number for elapsed time. Converting 1 day 5 hours 30 minutes into 29.5 hours by hand is where transcription errors creep in — and an error there propagates through every subsequent step.
How this calculator works
A difference, expressed two ways:
Difference in milliseconds = end − start
Total minutes = difference ÷ 60,000, rounded down
Days = minutes ÷ 1,440 · Hours = (minutes mod 1,440) ÷ 60 · Minutes = minutes mod 60
Total hours = difference ÷ 3,600,000The days, hours and minutes breakdown truncates seconds; the decimal hours figure retains them. For intervals measured in hours or days the difference is immaterial, but it is worth knowing the two figures are not derived identically.
Worked example: overnight and into the next afternoon
Using the default figures — 18 July 2026 at 09:00 to 19 July at 14:30:
- Interval: 1 day, 5 hours, 30 minutes
- Total hours: 29.50
29.5 hours is the figure that would go into an accumulation or elimination calculation. At a 15 mg/100ml per hour elimination rate that is 442.5 units of decline; at a 10°C temperature above threshold it is 295 accumulated degree hours. The interval itself is trivial arithmetic — its value is in feeding the next step without a transcription error.
Common mistakes
- Mixing time zones between the two values. Both are interpreted locally.
- Ignoring a daylight saving transition. Local time and elapsed time diverge across one.
- Entering the values in the wrong order. The calculator rejects it rather than showing a negative.
- Converting days and hours to decimal by hand. Use the figure provided.
- Assuming device clocks agree. Reconcile offsets before combining records.
Frequently asked questions
How are time zones handled?
The input fields use your device's local time zone, and both values are interpreted the same way, so the interval is correct as long as both refer to the same zone. Records originating in different zones need converting to a common basis first — UTC is the usual choice for anything shared between organisations.
What happens across a daylight saving change?
The elapsed time is what the calculator returns, based on the underlying timestamps. In the UK, an interval spanning the March transition has one fewer hour of local clock time than elapsed time suggests, and one more across the October transition. Recording in UTC avoids the ambiguity entirely.
Why is the total shown to two decimal places?
Because downstream calculations need a decimal figure, and two places resolves to well under a minute. Reporting an interval to that precision is rarely meaningful in itself, but carrying it into an accumulation calculation avoids a rounding step.
Can this handle intervals of months?
Yes, and it reports them in days. Months are deliberately not used as a unit because they vary in length, which makes them ambiguous for elapsed time. Days, hours and minutes are unambiguous.
Is what I enter stored?
No. Everything you enter is processed in your browser and is never transmitted or retained. Even so, treat any public website as an unsecured environment and do not enter identifiable case information.
Related tools
References
- GOV.UK — Forensic Science Regulator codes of practice on records and timekeeping
- Met Office — UK time and daylight saving reference information
Sources are checked at publication and can change — how I choose and check references.