I would start your seed indoors now for best results, then transplant out at 2nd or 3rd leaf (appx 5")
As for thyme and rosemary, real slow growers. I would have started seed indoors last fall, you might want to consider buying starter plants. They also don't care for hard, cold winters.
Our annual herb plants for market (May 24) are already 1-2" out of soil. First of the tomato plants at 5". All in greenhouse.
If insisting on outdoor seeding, look up the appx last date for frost in your area, and plant after that.