Japanese stiltgrass begins to germinate a couple of weeks before crabgrass.  The date for crabgrass is somewhat of a moving target because it depends upon soil temperatures, e.g. weather.  And since the spring weather varies from year to year, the germination date cannot be precise.  

This page explains crabgrass germination timing and soil temperature: https://extension.umd.edu/hgic/topics/crabgrass  

In general, crabgrass germinates about the time that forsythia blooms drop off.  https://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/non_HGIC_FS/TT-43e%20Crab.pdf

Our page on stiltgrass explains other tactics for management: https://extension.umd.edu/hgic/topics/japanese-stiltgrass

Ellen