[time-nuts] Smart Phone time display accuracy...?

[John Hawkinson]

There is a lot of...problematic...advice here. For instance:

Gary Woods <garygarlic at earthlink.net> wrote on Thu, 16 Mar 2017
at 18:09:26 -0400 in <u53mcc534t4tu924pc6rpqklolf7oelkp6 at 4ax.com>:

> Not to gloat, but my Android phone is always spot on.  I have a GPS
> time app that shows the difference between GPS and phone time and
> it's always in the tenths of a second area.

Notice Gary doesn't specify which brand of Android phone, much less
the specific model and carrier. This matters...a lot.

For instance, my Verizon Samsung Galaxy S7 is regularly off by 300-700ms,
and sometimes off by more than a second. A colleague's cell phone
is regularly off by about 10 seconds. On the other hand, another colleague's
Nexus {i forget what model} seems good to 10 ms.

It's not clear to me how Android phones set their own time from the
cell network, but I think the actual answer is "in many cases, not
very well."

The "ClockSync" android app will compare your phone's time to NTP,
which does a pretty decent job.
The "GPS Time" and other similar apps will compare your phone's time to
UTC as derived from your phone's GPS receiver, although I think this is
necessarily imprecise not only because of multipath but because of software
issues between the GPS receiver and the phone. *handwave*


I can't speak for iPhones, but for Android phones, it's really
extremely hit-or-miss, and highly variable, both between models and
even with a given phone from day to day. And if your threshold is
something like half a second, you might think your phone is always
good enough, and then have a rude awakening (I did!) when one day it's
off by >1 second.

--jhawk at mit.edu
  John Hawkinson