That’s okay, the forsythia don’t know the date either 
In my opinion, the forsythia rule of thumb is bad. By the time they begin to bloom, the temps are already ideal for germination. The fact is,the stuff lasts for 6-8 weeks when liquid is put down and 8 to 12 weeks when granular is put down. The reason we don’t put it down very early is because of runoff when the ground is frozen hard.
Now, I’m not saying I know anyone who does this, but technically you could put down liquid herbicide when overnight temps are reliably above freezing, like 40 degrees. There won’t be the runoff problem.
This is not advice to do that, but an explanation for why we don’t put down product when the ground is frozen.
If you get a pre-emergent like Dimension (Dithiopyr) down while crabgrass seedlings are still immature, it will still work. Not as well as if you got it down before germination (efficacy reduced by about 20%) , but it is the best defense against crabgrass.
If it is way late, you can opt for Tenacity or Drive, both use Mesotrione as the active ingredient and work as a pre AND post emergent. But the post is most effective on broadleaf weeds.