GDD is calculated using the sum of degree F above the Predetermined minumum temperature from the starting date up until the ripening date. Daily min and max temperature in degree F are retrieved from your hearest weather station.
If min of observed temperature is less than min, the min (50 degree F is default) is used.
If max of observed temperature is larger than max, max (86 degree is the default) is used.
The reset temperature is added as optional parameter. This is the temperature that the GDD will be reset to 0. I am adding this temperature to allow the calculator to account forthe freezing damage to the fig tree and the fig tree has to recover and restart the growing process. A warm Feburary won't contribute the GDD if there is a hard freeze in march.
Note that your weather station may not record the temperature every day. The previous day's temperature will be used if the termperature is not recorded that day. I am still in the process in evaluating the weather stationss to minimize the missing recording.
More information about GDD see PennStates Agriculture Extension Website
*record missing from NOAA, value is taken from previous day