Time slowly goes out of sync Cygwin+windows+psotgres

Running postgres on cygwin and windows.

Cygwin has the correct time.
Windows has the same time as cygwin.

Postgres slowly goes out of sync.  Over a month it ends up 10 minutes out.

using e.g. "SELECT NOW()";

restart the machine and posgres is only 30 seconds out.

Is postgres calculateing the time itself or reading of the machines clock?

Is there a setting to make it go off the operating system clock?