The warranty clock starts on the day of the first retail purchase. You need proof of this if you ever wish to get warranty work done. If there is no proof of first retail purchase, the warranty clock starts on the day the unit was invoiced to the dealer (or Home Depot). If you can share the serial number with me, I can tell you if there is any warranty remaining on the unit.
If you buy a new HRR or HRX between now and May 1, AND REGISTER IT WITH HONDA, you will get +1 extra year of factory warranty. To register, fill out and drop the postage-paid card (that is in the box with the new mower) in the mail, or go here with the serial number and register it online:
Register Your New Honda Online