Timesheets seem straightforward until the moment you try to add up 7 hours 45 minutes, 8 hours 20 minutes, and 6 hours 50 minutes and discover that standard arithmetic produces the wrong answer. This is because time uses a base-60 system (60 seconds per minute, 60 minutes per hour) while regular arithmetic uses base-10. Treating time values as ordinary numbers produces errors that range from mildly annoying on a personal invoice to seriously problematic on a payroll for a team of staff.
The Fundamental Problem With Adding Time
Here's the error in action. Adding three work periods: 7h 45m + 8h 20m + 6h 50m.
Wrong approach (treating as regular numbers): 45 + 20 + 50 = 115 minutes; hours 7 + 8 + 6 = 21 hours; "total" = 21h 115m. Incorrect — there's no such thing as 115 minutes in a time expression.
Correct approach: 115 minutes = 1 hour 55 minutes (115 ÷ 60 = 1 remainder 55). Total = 21 hours + 1 hour 55 minutes = 22 hours 55 minutes. The correct answer is nearly an hour more than the naive calculation suggests if you were to misread "21h 115m" as "21h 15m."
Use our time duration calculator to add, subtract, and total time values correctly. It handles the base-60 conversions automatically, so you can enter hours and minutes as you record them and receive accurate totals without manual conversion arithmetic.
Decimal Hours: The Format Payroll Systems Need
Most payroll software, invoicing systems, and billing platforms expect time entered in decimal hours rather than hours:minutes format. Decimal hours express the minutes as a fraction of an hour rather than as a separate minute count.
Conversion formula: Decimal hours = Hours + (Minutes ÷ 60)
- 7h 45m = 7 + (45 ÷ 60) = 7 + 0.75 = 7.75 hours
- 8h 20m = 8 + (20 ÷ 60) = 8 + 0.333 = 8.33 hours
- 6h 50m = 6 + (50 ÷ 60) = 6 + 0.833 = 6.83 hours
- Total: 7.75 + 8.33 + 6.83 = 22.91 hours
To convert back: 22.91 hours → 22 hours + (0.91 × 60) = 22 hours 54.6 minutes ≈ 22 hours 55 minutes. The tiny rounding difference is negligible for payroll purposes.
Once in decimal hours, multiplying by an hourly rate gives earnings directly: 22.91 hours × £35/hour = £801.85. Use our salary to hourly calculator to work backwards from an annual salary to an equivalent hourly rate for contract billing or comparison purposes.
Handling Start and End Times
When recording by start and end time rather than duration, subtraction is needed first. The same base-60 issue applies. For a shift starting 08:45 and ending 17:20: simple subtraction of 1745 − 0845 in decimal gives 900 (minutes), which is wrong. Correct approach: hours difference = 17 − 8 = 9; minutes difference = 20 − 45 = −25 (negative, so borrow an hour); adjusted = 8 hours 60 + 20 − 45 = 8 hours 35 minutes.
For overnight shifts — starting one day and ending the next — add 24 hours to the end time before subtracting. A shift from 22:30 to 06:15: end time in 24+ format = 30:15. Duration = 30:15 − 22:30 = 7 hours 45 minutes.
Recording Part Days and Breaks
Unpaid break deductions are the most common source of timesheet disputes. Best practice: record total time at work (start to end), then record break time separately, then subtract. Entering start time, end time, and break duration separately — rather than calculating net working time yourself before entering it — provides a clear audit trail and makes any discrepancy between recorded and paid time immediately visible.
For contracted hours monitoring (checking whether an employee has met their weekly hours commitment), the sum of daily net hours minus contracted hours gives the surplus or deficit for the week. This calculation is what our time duration calculator and scheduling calculator automate — particularly useful for flexible working arrangements where hours are tracked against a weekly total rather than fixed daily schedules.
Timesheet Records: Legal Requirements
In the UK, employers are legally required to keep adequate records to demonstrate compliance with the National Minimum Wage. This effectively means keeping timesheet records showing hours worked for all workers paid at or near the minimum wage. Records must be retained for at least three years.
For freelancers and contractors, detailed timesheets serve as the evidence base for invoices and protect against payment disputes. Recording project codes, client names, and task descriptions alongside time totals transforms a timesheet from a billing document into a business intelligence record that informs future project pricing.
The GOV.UK guidance on National Minimum Wage record keeping requirements sets out what records employers must maintain and for how long, which is relevant context for any business managing hourly-paid staff timesheets.