Select "crabgrass PRE" on the right side and it will show a map for your area with early, optimum and late timing. The model was developed for the Michigan area, but should work OK for the northern half of the country. It's showing NJ is just entering the optimum period and will be there for maybe the next ten days or so. Another general guide is to put it down when Forsythias bloom, that hasn't started here yet.
I use Dimension, which will also kill very early crabgrass plants, so I time mine near the end of the optimal range, keeping rain in mind as it needs a rain to water it into the soil and make it effective. If you have crabgrass troubles most years, a second application about 6 weeks later is a good idea too.