If you have a local weather station that records the daily temps, get their last several years records, and enter them into a spread sheet.
Threshold = 7
Sept High Low Hours
1 15 6
2 20 11
3 10 2
If (Low >= threshold) hours = 0
Else hours = 24 * (Threshold - Low)/(High - Low)
For Sept 1 you have 24 * (7-6)/(15-6) = 2.something
For Sept 2 it didn't get cold enough = 0
For Sept 3 you have 24 (7-2)/(10-2) = 15
This is an approximation that assumes that the high and the low are about 12 hours apart. Usually the low is about dawn, and the high in mid afternoon, so this under estimates chilling hours somewhat.
If you want degree days, then replace 24 by 1.
It's near enough for most purposes. Chilling requirements aren't that precisely known. And for most things, different amounts of chill have different effects. For some chilling only counts when it's between to values, say -2 and +6. When it's colder than that the 'clock' freezes.
Chilling also varies by cultivar for apples. Growing apples on the Canadian Prairies required breeding for higher chilling requirements to prevent late frosts from killing all the buds.