Keep in mind when planting that Sierra Sunset is thought of as hardy, so it can be safe to leave outdoors for the majority of winter (although if in doubt, using a row cover is often a good idea).
By our calculations, you should look at sowing Sierra Sunset about 49 days before your last frost date.
As Sierra Sunset is hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.