Insanely ridiculous timeline
The 1st series according to the title credits starts in 1955.
It is stated the Airmen will do 8 weeks basic training.
They spend Xmas day 1955 at RAF Skelton.
They then leave and go to RAF Medham for 13 weeks in series 3 & 4.
After the end of their 13 weeks at RAF Medham its STILL 1955.
Cpl Marsh goes to Canada.
They then spend a week in Malta before being posted to RAF Druidswater where oddly enough its STILL 1955.
Now to add insult to injury they meet up with Cpl Marsh who it is said rescued an Officer in Canada on 14th November 1955 in series 5.
Which is a damn good trick considering he'd have still been training National Servicemen as a drill instructor at RAF Skelton prior to series 1 at the same time.