I've never tried to seed it outside, but I have started thyme seed inside before transplanting. It's worked great for me. Germinates in a few days and grows fast.

"Mother of Thyme" Thymus serphyllum is low and easy to grow from seed. I start my thyme 1st week of February. Grow in small pots or plug tray. Use a soiless medium and barely cover the seeds with builders sand or fine vermiculite. Water from bottom and do not over water. When you sow put in about 10-15 seeds per plug/pot so you have a nice bushy plant. They will germinate in about 8-10 days in a temperature of 65-70 degrees. It can be grown outside but make sure you sow your seed on a thick layer(2-3")of a soiles medium(Pro-Mix)or any other brand you garden center might have. You need this so your seeds are not competing with the germination of weed seeds. If this is still a problem you can wait till you temperature warms up and sow your seeds in large rectangular plastic storage containers. Punch holes in bottom for drainage. Fill container(3-4")with soiless medium and generously sow seeds. Place in a partially shaded area and mist when needed. When container is full of nice size seedlings remove small clumbs to the area you want to cover.

It might be hard to see but this is my thyme garden. There are many differend varieties of thyme. Each mound was 3 small plugs/pots and this garden is only 1 year old. Thyme spreads very fast. The hot pink flower is silene mexicana which is an annual but reseeds for me every year.