A valuable tool for farmers is the Degree Days system in which you calculate a number based on temperature patterns over a set value. Degree days are tracked by ag extension offices which makes accessing local calculators very easy.
Degree days have more usefulness IMO than day length for determining plant activity such as bud and fruit sets, and most importantly managing an IPM system.
https://en.wikipedia.org/wiki/Degree_day
http://ipm.ucanr.edu/WEATHER/ddconcepts.html
http://jenny.tfrec.wsu.edu/opm/displaySpecies.php?pn=-50
