The optimal time to plant carrots in zone 7a is usually during the cool spring or fall season when the soil temperatures are around 50°F (10°C). However, with proper care and attention, it may still be possible to plant carrots later in the growing season if there is enough time before the first frost. Carrots have a relatively long growing period of about 70-80 days from seed to harvest, so it's essential to check the estimated days to maturity for the carrot variety you intend to grow. If you're nearing the average first frost date in your area and the carrots will not have enough time to mature, it might be better to wait until the next planting window.