Unfortunately, some clergymen used them in earlier registers. The system works like this. Instead of a normal calendar year, the year written down was the number of years since the year in which the Coronation took place. Worse still, year number 1 started on the day of the Coronation. Often, but not always, the day the previous monarch died.
Years alone are relatively easy to calculate.
The ones we are really interested in are those from 1538, when parish registers began, although of course Regnal years were commonly used before this date. It is uncommon for Regnal years to be used much later than this. But there was still the occasional die-hard clergyman who did, and some scholars and historians still use Regnal years.