Per 400 years, there is one leap day every 4 years (100 leap days), except when the year is divisible by 100 (so we overcounted by 4 and there are 100 – 4 = 96 leap days), except when the year is divisible by 400 (so we need to add that day back and arrive at 100 – 4 + 1 = 97). This gives us 97/400 = 0·2425.
The tropical year is about 365·24219 days long, but that's not relevant to timekeeping.
Per 400 years, there is one leap day every 4 years (100 leap days), except when the year is divisible by 100 (so we overcounted by 4 and there are 100 – 4 = 96 leap days), except when the year is divisible by 400 (so we need to add that day back and arrive at 100 – 4 + 1 = 97). This gives us 97/400 = 0·2425.
The tropical year is about 365·24219 days long, but that's not relevant to timekeeping.