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Phones usually just use network time, because the GPS is an expensive component to spin up.

Can you explain 'spin up'?

Do you mean that its Power/battery intense?

Yup. The GPS drains battery.

But when GPS is going for another purpose phones could grab the time. Don't know if they do.

Initial GPS signal acquisition is power intensive, but if it is already tracking, then there should be no incremental cost to read the time information.

GPS time tends to be very precise as well. In the NTP world, (simplifying greatly) GPS can serve as a reliable “stratum 0” time source which feeds stratum 1 servers, which go on to feed the rest of the NTP network.

Building accurate NTP stratum 0 GPS time source is somewhat nontrivial endeavour. Which explains why most of stratum 1 NTP timeservers (typically GPS based) give wrong time (with error on the order of 500ms).

But what value would that add? How often would you need to sync your phone clock but not be connected to network (which will be providing time sync anyways)?

If you're off the grid somewhere. Or just a cool bragging right.

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