We set the growing degree day (GDD) clock to ZERO today, April 1st, when we begin accumulating GDD base 50 from now on. April 1 is usually when grape buds begin to expand, although it can be earlier or later in any given year.\

http://agebb.missouri.edu/weather/reports/gddTable.asp

Phenology stages according to the Modified E-L system
http://door.uwex.edu/files/2010/10/ModifiedEichhornLorennzsystem.pdf