Because the small internal cell that keeps the clock running has discharged, and it charges only while the main battery is in the camera. A body left empty for months forgets everything the clock holds.
This is one of the few faults with a single likely cause and a free first test.
The mechanism
A camera keeps a separate small cell for its clock, so the time survives a battery change. That cell is charged from the main battery while it sits in the body. Leave the camera empty long enough and the cell runs down; then every insertion looks like a first power-up.
What to try, in order
- Leave a charged battery in the body for a day or two, then remove it briefly and check whether the clock survived.
- Set the date, then power-cycle to confirm it persists at all.
- If it does not, it is a service question — the cell is internal on most designs.
Why the setup list puts the clock first
Because every file carries the timestamp and no later correction is as good as having had it right. Along with the file format and formatting the card in the camera, it is one of the settings whose absence costs a whole outing and whose presence costs a minute.
What no specification covers
How long that cell holds without a main battery. No listing publishes it, and the practical answer is the habit: a charged battery left in the body between outings.
What people ask before they commit
Can I replace that cell?
On most bodies it is internal and not user-serviceable, which makes this a service matter rather than a purchase. The first thing to try costs nothing: leave a charged main battery in the camera for a day or two, because in many designs that is what recharges the clock cell.
Does it matter beyond the date stamp?
It matters more than it looks. The date orders your files, and a set shot across two days with a reset clock in the middle is a set that will not sort. Anything that pairs frames by time — a second camera, a recorder, a flash log — also depends on it.
Last reviewed 12 September 2026