Calculating time worked over the years (6)
I believe the answer is:
shifty
'calculating' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for shifty that I've seen before include "Evasive, sneaky" , "Suspicious-looking" , "Sly, shady" , "Furtive" , "Constantly changing; deceitful, evasive" .)