The daily number is close, but never equal
The warehouse daily revenue figure and the finance system's daily figure differ by a small amount every day. Neither is drifting, the totals over a month are nearly identical, and no check has ever fired.
What you would do first
Answer before revealing anything. The value of the exercise is entirely in committing to a diagnosis you can be wrong about.
- 1Compare the two figures for a single day and then for a whole month. A gap that mostly disappears over a month, with daily differences that alternate in sign, is a boundary problem rather than a loss.
- 2Group the warehouse figure by hour and look at the first and last few hours of the day. The difference will be concentrated there.
- 3Read the column being used for attribution and find out what time zone it is stored in and what time zone it is being truncated to.
- 4Confirm with finance which boundary is authoritative for reporting. This is a policy question, not a technical one.
What is actually going on
The trap
The fix that looks right. Read it even if you got the answer — especially then.
Convert the entire warehouse to the reporting time zone so the numbers match. Every timestamp is now ambiguous twice a year at the daylight-saving boundary, every join to a system that stayed on UTC is subtly wrong, and the original UTC value — the only unambiguous one — is gone.