When were leap years prior to 1752?

As far as I know the Julian calendar calculated leap years the same way as the Gregorian, ie the year divisible by 4 except for centenial years which had to be divisible by 400.

So 29 Feb 1672 would be a valid date in either calendar. If the date was written in a Parish Register on the Island it should appear just before the entries for, say, 29 Mar 1673.

This would mean that on the Island leap year was a year behind the continent.

I'm trying to make sense of entries in the transcripts of the Braddan Burial Register.
What does the date of 29 Feb 1673 mean.
I'll assume transcription error for 30 Feb 1638.