HF and VHF. Which do we use, and when?

[aterpster]

con-pilot:

You have a Rolex watch don't you? Every pilot that I flew with that had a Rolex watch checked their watch with the HF, frequency usually 15.000, every time they got into the cockpit and then reset their Rolex to the correct time. My Seiko Mickey Mouse watch never lost a second.

I have an inexpensive Seiko that syncs with WWV every night.