Two Clocks, One Alibi
A gold-ornament shop 2.4 km inside Nepalgunj was robbed at 19:45 on 14 April; the shop's CCTV clock runs 3 minutes fast. The suspect, a driver, says he left a wedding at Kohalpur at 19:20, rode his motorcycle homeward at about 40 km/h, and stopped for fuel at a pump, where he holds a printed receipt timed 19:38. Investigators have established that the pump's CCTV clock is 6 minutes slow, but they have not checked the clock of the pump's billing printer that generated the receipt. A highway ANPR camera 2.4 km before the shop, whose server clock is network-synced and reliable, logged his number plate at 19:31. The road is a single highway running Kohalpur - 13 km - pump - 10.6 km - ANPR point - 2.4 km - shop. A shop employee recalls a red motorcycle and a black helmet near the door at about the time of the robbery but did not note the number plate. The suspect's motorcycle is red and his helmet is black.
1. Rebuild the timeline with the clock corrections and decide whether the suspect could have been at the shop at 19:45. Show the arithmetic.
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Corrected times. The shop camera runs 3 minutes fast, so the 19:45 robbery was recorded as 19:48 - this matters when matching any shop footage. The ANPR server is synced, so 19:31 is 19:31.
From the ANPR point to the shop. 2.4 km at 40 km/h = 2.4 ÷ 40 × 60 = 3.6 minutes. Arrival at the shop about 19:34:36, i.e. ten minutes before the robbery. He had the opportunity to wait, enter and leave.
Checking the alibi's own numbers. Kohalpur to the pump is 13 km = 19.5 minutes at 40 km/h. If he left at 19:20 he would reach the pump at about 19:39:30 - very close to the 19:38 receipt, which is why the alibi looks plausible at first. But the pump is 13 km before the ANPR point: 13 ÷ 40 × 60 = 19.5 minutes, so a genuine 19:38 refuelling puts him at the ANPR point at about 19:57:30, not 19:31.
Conclusion. The receipt and the ANPR log cannot both describe one rider travelling in that direction. Since the ANPR clock is the one that has been verified, the receipt is the weak item - and the printer clock has not been checked. On the verified record the alibi fails: the motorcycle was 3.6 minutes from the shop ten minutes before the robbery.
2. Which assumptions could overturn that conclusion, and what would you check?
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- The printer clock. If the billing printer runs the same 6 minutes slow as the pump CCTV, the 19:38 receipt is really 19:44 - which is after the ANPR sighting and therefore impossible on the stated route. Either way the receipt does not rescue the alibi until the printer's clock is tested against a known time signal and its transaction log pulled for the whole evening.
- Identity of the rider. The ANPR proves a plate, not a person. Check helmet colour in the image, whether the capture is front or rear, and whether another family member uses the bike.
- The plate itself. A single misread digit produces a false match; obtain the full-resolution image and a second camera if one exists.
- Coverage and direction. Does the camera cover all lanes, and does the log record direction? A vehicle heading the other way is not corroboration.
- Alternative routes. If a bypass joins between the pump and the shop, the 19.5-minute leg need not hold.
- Speed. He may have ridden faster or slower than 40 km/h; run the leg at 30 and 50 km/h and see whether the conclusion survives.
The honest position after the arithmetic is: opportunity is established, guilt is not.
3. What investigative steps would turn this timeline into evidence a prosecutor can use?
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- Seize and preserve the records, not copies: the ANPR image file with its hash and export log, the pump's transaction list for 18:30–20:30 on 14 April, and the pump's printer clock photograph alongside a network time reference.
- Test the clock question directly by comparing the printed receipt time with the transaction recorded in the pump's system and with the CCTV frame of the same customer at the counter.
- Link rider to bike to person: shop footage corrected by −3 minutes, helmet colour and any distinctive clothing, and the motorcycle's seizure with photographs of the damage or accessories visible on camera.
- Fuel-pump attendant statement - who paid, in cash or card, and whether the receipt was issued to this customer.
- Phone records for the 19:20–20:00 window, with the cell sector of any call, as corroboration only; a tower sector is not a location.
- A controlled timing run along the same route at the same hour, recorded, to fix the realistic travel time rather than an assumed 40 km/h.
- A timeline chart in the file showing each event in verified time with its source, so the prosecutor can see which links rest on synced clocks and which do not.