Dawn time shouldn't be messed with, it WILL add stress to your plants. But if it's inevitable, then do it.
Figure that out on your next grow, lights shouldn't be on at night anyway, not stealthy at all.
If your veg schedule looks like: On 6 am, off 12am.
then your flower schedule should be:
On 6 am, off 6 pm. Keep the dawn time consistent.