Actually, some of them do have dates on. For example the one at the bottom of hillside at the junction with the avenue has dates on. It lists the next two dates the scheme is in place. That is the start of the parking restriction zone, the signs in my street don't have dates on, so it appears to just be signs on the main entries to the restricted area that have dates.
I stand corrected: the ones in the streets around me don't have any dates on and I'd not seen any that had, but then, I'm not in Moulsecoomb much