Typical strip chart showing phase drift. In this example the local standard being compared has drifted about -3 µs over a span of 7 hours, for a frequency offset error of about -3 µs / 7 h = -1.2 x 10-10.
Every hour, at 10 minutes past the hour, WWVB shifts phase by 45° for 5 minutes. At 60 kHz (16.67 µs), that's about 2 µs. Here's a close-up:
Five days of paper. Note that there are many times (at least here in Seattle) when reception is poor.
Below is an example of either poor reception or injection locking. The lack of WWVB phase signature on the left means there is either no signal or a signal is being picked up from some nearby electronics.