From this derived the practise of allowing 2 minute departure separations to consist of 62 seconds actual spacing e.g. No1 departs at 1215:29 (=1215) and No2 at 1216:31 (=1217)! Useful to tower controllers in a tight corner but frightening to departure radar who would thereafter insist on 120 seconds between deps.
Ah, the old days.....