There are several possible ways:
- Timepiece. A clock or watch which has been calibrated before heading offshore. Even better, if you know the drift rate of the timepiece, you can compensate for drift error after departing.
- Shortwave (HF) radio. There are several radio stations broadcasting the time long distance. The U.S. stations can be found by tuning in AM to 2.5, 5.0, 10.0, 15.0, or 20.0 MHz on a shortwave or HF radio. You can also hear it by dialing +1 808-335-4363, presuming you have a phone.
- GPS. GPS units will give you very accurate time.
- Sextant. A sailor can use a sextant to observe when the sun reaches its apex during the day, known as “local apparent noon.” You may wish to simply declare this to be “noon” and call it good enough. But if you also know your approximate longitude, it’s possible to compute a more precise time.