do not calculate a time diff against 1.1.1970

nitpicking, but ... sending a "USR1" to a tor relay here gives

Nov 06 17:51:15.000 [notice]  * Channel 17537 was created at 1604681438 (37 seconds ago) and last active at 1604681438 (37 seconds ago)
Nov 06 17:51:15.000 [notice]  * Channel 17537 was last used by a client at 0 (1604681475 seconds ago)
Nov 06 17:51:15.000 [notice]  * Channel 17537 last received a cell at 0 (1604681475 seconds ago)
Nov 06 17:51:15.000 [notice]  * Channel 17537 last transmitted a cell at 0 (1604681475 seconds ago)

The time diff calculation gives no further information here, right? (FWIW: git-dd119b27)