Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Tampa to Guam as you normally would.
Due to multiple factors, expect to potentially pay more when booking in February. For low rates, May can be a great choice for travel to Guam. Flights to Guam from Tampa in May are about $1,784 on average during May, but can be found for as low as $1,004.