By our calculations, you should look at sowing Amsterdam about 14 days before your last frost date.
Sow in deep, well-worked, stone-free soil after danger of heavy frost from spring to midsummer. Do not transplant, as crooked roots may result. Cultivate periodically or mulch to control weeds. Keep soil evenly moist.
As Amsterdam is very hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.By our calculations, you should look at planting out Amsterdam about 14 days before your last frost date.