If you have no Internet access, you can use a "Hockey puck" GPS device to accurately set the time IF you have a real hardware serial port on your PC (does not work with USB-to-Serial adapters). The simplest method, with documentation:
(Also works without hockey puck).
Correction: I was thinking of Faros, which syncs time with several internet time servers, or with ToyNTP. ToyNTP itself only works with a hardware GPS on a real serial port.
The Red Pitaya time and frequency can also be calibrated using the same GPS hardware, as explained in Pavel's documentation, under "GPS Interface"
I asked him if a GPS could also be used to calibrate sdr_receiver_hpsdr. Unfortunately, no. He said that would be a non-trivial enhancement.
73,
Bob, N6TV