If you are talking about after 1500 on a reserve day then yes. An A reserve is on call 24 hours a day during his reserve block. He can be called at any time and given a 9-hour break for a B reserve callout window. For some commuters A reserve is essentially useless. Watching TV in Florida at 10pm, phone rings and told you're converted to B reserve in EWR with a callout of 7am. You're hosed.

If you're talking about after 1500 on the day off before a reserve day, then no, as flybynuts said.