For counters, timestamps are automatically computed to be milliseconds since the epoch. For everything else, when not specified manually, they are microseconds since the epoch. This means if you delete a counter row, subsequent updates are lost unexpectedly.
I know that deleting counters is not recommended, but that's only because deletes and incs don't commute. If you know you have stopped incs, then delete, then start again (with some external synchronization) then deleting is fine.