How can an STM32 logger keep accurate timestamps across low-power intervals?

Identify which clocks and counters continue in the selected low-power mode on the exact MCU. A CPU-driven tick may not measure the sleeping interval. Use a suitable retained timebase for elapsed or calendar time and document its resolution, tolerance and reset behavior.

Timestamp the measurement event, not merely the later file write. Include sensor settling and conversion delay when interpreting the timestamp. If an alarm schedules wakeup at a given time, the sample is not necessarily acquired at that instant; record or bound the intervening latency.

Test repeated sleep cycles, oscillator startup and power loss. Distinguish monotonic elapsed time from calendar time that may be corrected by synchronization. Preserve sequence numbers so a clock correction cannot hide missing or reordered samples, and quantify drift against the application's timing requirement.