There is time-sensitive information in the NAV message in both subframe 1 and subframe 4. The information in subframe 4 helps a receiver relate two different time standards to one another. One of them is GPS Time and the other is Coordinated Universal Time (UTC).
GPS Time is the time standard of the GPS system. It is also known as GPS System Time (GPST). Coordinated Universal Time is the time standard for the world. The rates of these two standards are virtually the same. Specifically, the rate of GPS Time is kept within 1 microsecond, and usually less than 25 nanoseconds, of the rate of Coordinated Universal Time (UTC) as it is determined by the U.S. Naval Observatory Master Clock (USNO MC). The exact difference is in two constants, A0 and A1 in the NAV message which give the time difference and rate of system time against UTC.
The rate of UTC itself is carefully determined. It is steered by about 65 timing laboratories and hundreds of atomic clocks around the world and is remarkable in its stability. In fact, it is more stable than the rotation of the earth itself, such that UTC and the rotation gradually get out of sync with one another. Therefore, in order to keep the discrepancy between UTC and the earth’s actual motion under 0.9 seconds, corrections of 1 second, called leap seconds, are periodically introduced into UTC. In other words, the rate of UTC is consistent and stable all the time, but the numbers denoting the moment of time changes whenever a 1 second leap second is introduced.
However, leap seconds are not used in GPS Time. It is a continuous time scale. Nevertheless, there was a moment when GPS Time was identical to UTC. It was midnight, January 6, 1980. Since then, many leap seconds have been added to UTC, but none have been added to GPS Time. So, even though their rates are virtually identical, the numbers expressing a particular instant in GPS Time are different by some seconds from the numbers expressing the same instant in UTC. For example, GPS time was 16 seconds ahead of UTC on July 1, 2012.
Information in subframe 4 of the NAV message includes the relationship between GPS time and UTC, and it also notes future scheduled leap seconds. In this area, subframe 4 can accommodate 8 bits, 255 leap seconds, which should suffice until about 2330. The NAV message also contains information the receiver needs to come close to correlating its clock with that of the clock on the satellite. But because the time relationships in GPS are changing constantly, they can only be partially defined in these subframes. It takes more than a portion of the NAV message to define those relationships to the necessary degree of accuracy.
Since we've talked some about time, it probably is a good moment to discuss the time standard that is used with GPS. Worldwide, the Coordinated Universal Time, or UTC standard is well known. It is driven by several atomic clocks around the surface of the Earth. However, Coordinated Universal Time, as derived from these clocks, is in some sense more accurate than the actual rotation of the planet itself. Therefore, the rotation of the Earth and UTC get out of sync with each other. To keep UTC connected to the planet, leap seconds are added to Coordinated Universal Time periodically. Since GPS is space-based, the decision was made that GPS time would have the same rate as UTC, but it would not use leap seconds. So, on January 6, 1980, it was decided that GPS time and UTC would be exactly together. They would be the same. But since then, there have been leap seconds added. That is complicating the relationship a little bit. The most important thing to remember is that the rate of GPS time is intended to be the same as the rate of UTC.
The Control Segment keeps track of time and uploads clock corrections to the satellites. It's available to your receiver in the Navigation Message. They try to keep the rate of GPS time at the satellite within 1 microsecond of the rate of UTC. Of course, the oscillators (clocks) in satellites have a tendency to drift. They're not perfect timekeepers, and they're affected by several things, moving in and out of the shadow of the Earth, gravitational changes, and so on. These effects cause the satellite's clocks to not oscillate perfectly.
The Control Segment does not constantly tweak the clocks in the satellites to keep them perfectly on GPS time. They let them wander a little bit within the limit of one microsecond. They find that by doing this, it keeps the clock in the satellites healthy longer. It extends their life. You will note that the first things to go on a satellite are its clocks. This is one reason that many of them have two, a cesium and a rubidium oscillator.
So, the message is that the Control Segment introduces a clock correction into the Navigation Message that gives your receiver a way to correlate the drift of the oscillator(s) in the satellite with GPS time. This is one way to keep the time standard within limits.