What do you do on a partly cloudy day? Use a watch? how do you correct for seasonal variations? and the point that one set of va lues is a subset of another larger set of values... seems most contradictory on the surface....
Besides it's a most poor method. Perhaps you could start a timer triggered at the first occurrence of the "inside" set of values.
Might be easier to start a timer to detect first light and add a table of first light vs season Times for first light... that approach might have a chance of working... after a "fashion" you could use the timer to report the time of day... although an RTC just Might be the wisest choice... your approach seems similar to using a liter measure to accurately measure half liter quantities...
Doc