This hardly rates as a problem. National Service might have been abolished in the 4 years before Patrick became 18 even if the books had a realistic time scheme.

The time settings didn't bother me until 'Attic Term'; probably because lumping together any time after the war wasn't a problem until the books entered my lifetime.

The biggest problem I have with the timing is that Ginty has to phone the operator to get through to Patrick and it is the operator who gives away the fact that she's been phoning every night. That didn't strike me as realistic for the 1970s.