HTC may not be too terribly specific about when it intends to release its newest flagship, aside from a vague March-ish timeline, but Vodafone and Clove are more than happy to fill in the blank space: March 15th is the date. At least if you're in the UK, looking to get this piece of hardware on Vodafone, or order from Clove.
Not only does Clove's ordering site indicate this as the date the first shipments should come in, but Vodafone confirmed the date in a statement to AP. Pre-orders for the carrier's version will begin two weeks prior to release. To wit:
Just a quick note to confirm that Vodafone will be offering the HTC One for sale from 15th March 2013, with pre-order opening two weeks before.