I think that the clock starts ticking from it's first flight. When it goes into service is a matter of delivery, any changes made by the buyer, country getting over-run by an enemy and so on...those situations can disproportionately skew an aircraft's sevice timeline.
And I totally forgot about the Vildebeast...good stuff guys, we're finding some really old stuff here!
The TB-1 was older, and used for transportation duties until 1947. Some of them flying in the front vicinity, were rearmed.
Until 1945 there were some Polikarpov R-5 ( 1928 ) in the VVS inventory and thousands of Po-2 (1927) night bombers!!!