What is the PPM of the clock oscillator in a smartphone?
ppm error on the oscillator may vary from one model to another.  Here are some ballpark numbers.


simple crystal oscillator. 5 to 20 ppm
TCXO (temperature compensated crystal oscillator). 1 to 3 ppm drift, typically.


Calculating error envelope from ppm is easy.  1 ppm is 1 microsecond.  In 1 second, a 20 ppm clock can gain or loose 20 μs in the worst case.

On a smart phone, can you access the GPS time?  It originates from atomic clocks.  GPS time has good long term stability.  Just an $0.02 thought.

Another somewhat related thread.