Mint Seeds Not Growing

I recently bought some mint seeds online. I planted them in composted seed trays about a month ago with no sign of life!The package advises to sow in March but would this just be to avoid potential frosts? The variety I bought were ‘Tagetus Minuta Muster John Henry Peruvian Black Mint’ (a bit of a mouthful!). To me that sounds like that this variety grows only in hotter climates? I understand that they’re fairly aggressive once they take off.I would greatly appreciate if you could advise if I’m using the correct variety for this part of the world. As a general question re mint,how do they fare in Winter times?