Date variables help you define a date or date range you're interested in.
Find assets with last check-in between January 1st 2016 and December 31st 2016 (i.e. yyyy)
lastCheckedIn: "2016"
Find assets with last check-in between February 1st and 29th 2016 (i.e. yyyy-MM)
lastCheckedIn: "2016-02"
Find assets with last check-in anytime on February 1st 2016 (i.e. yyyy-MM-dd)
lastCheckedIn: "2016-02-01"
Find assets with last check-in on February 1st 2016 from 10 to 11am (i.e. yyyy-MM-dd HH)
lastCheckedIn: "2016-02-01 10"
Find assets with last check-in on February 1st 2016 at 10:15am (i.e. yyyy-MM-dd HH:mm)
lastCheckedIn: "2016-02-01 10:15"
Find assets with last check-in on February 1st 2016 at 10:15:44am (i.e. yyyy-MM-dd HH:mm:ss)
lastCheckedIn: "2016-02-01 10:15:44"