Enter both clocks to measure the offset.
The answer appears here.
Worked example
You check the recorder at 22:10 by your phone, which syncs its time from the network. The recorder's own display reads 21:57. The recorder is therefore 13 minutes slow. The incident you need was reported at 20:30, so on the recorder it will appear at 20:17. If the incident had happened at 00:05 on 4 November, the same offset gives 23:52 on 3 November, the night before, which is the kind of rollover people miss when scrolling a timeline.
The sign matters. Recorder minus actual gives the offset: a negative number means a slow recorder and you search earlier, a positive number means a fast recorder and you search later. In reverse, when you already have footage and need its real time, the same offset is subtracted instead.
Why it is an estimate
- The offset is only as good as the reference clock. Use a network-synced phone or a radio clock, not a wall clock or a wristwatch.
- Drift changes over time. A recorder that is 13 minutes slow today may have been 11 minutes slow a fortnight ago. For old footage, say so in your notes.
- Clock changes. Some recorders switch between GMT and BST automatically, some do not, and some switch at the wrong moment. If the check and the incident fall on different sides of a change, the tool flags it. On the last Sunday in October the hour from 01:00 to 02:00 happens twice; confirm which pass you are looking at.
- Record the offset in your pocket notebook and in the incident log, with the time you checked it. It becomes part of the evidential audit trail.
The Home Office and Forensic Science Regulator guidance on retrieving footage is on GOV.UK: Recovery and acquisition of video evidence. Time and date accuracy, and recording any discrepancy, is one of the first checks it describes. This is part of what the control-room practical on the SIA CCTV Operator course assesses; see our CCTV practical guide.
FAQ
How do I work out the time difference on a CCTV recorder?
Check the recorder clock against a reliable clock at the same moment. Recorder time minus real time is the offset. Add that offset to the real time of the incident to find where it sits on the recorder.
What if the recorder is fast rather than slow?
The offset is positive, so you search later than the real time. A recorder 5 minutes fast shows a 20:30 incident at 20:35.
Does the calculator handle the clocks going back?
It warns you when the times span a UK clock change or fall in the repeated hour, because recorders handle this differently. It does not guess what the recorder did; you need to check.
Can I use this for the reverse: footage time to real time?
Yes. Switch to reverse lookup, keep the same calibration, and enter the time shown on the footage. The result is the estimated real time.