{{ YEAR($NOW) }} failed upon updated my item

[weilies]

I tried a simple numeric field, get Year from a DateTime field, and store into a float field but nothing updated

Thanks for the support

[weilies]

my main purpose is to derive "hours_spent" value, based on 2 standard fields
time_start (Datetime)
time_end (Datetime)
hours_spent (computed field), with template below, it always return 0