What's the time?

[geoid96]

As already said, a GPS receive will show the time in UTC, using the leap second correction at the time the firmware was put together - which may be a few years ago, so you could look up the difference between then and now if you wanted to.

The GPS to UTC time correction is not embedded in the receiver firmware. It is transmitted by each of the GPS satellites on a regular basis in subframe 4 of the navigation message. With the last UTC leap second at the end of last year, the offset increased from 14 to 15 seconds and if you happened to be watching your GPS clock at that time it should have displayed 23:59:59 - 23:59:60 - 00:00:00.

Detailed description of signals broadcast by the GPS satellites can be found in the GPS Interface Control Document

Technically you should use UT1 (mean solar time) for astro observations not UTC. However the difference will always be less than a second and the precision of a sextant reading is never going to approach 1" arc. For astro at sea, the time from a reasonably adjusted Casio watch will be more than adequate.