when I type timedatectl I can see that sync is active. however the pi fails to get the time from a PC connected to it (on this pc the service W32time is running and should provide time info to all connected clients, this was configured as previously suggested on other forums, see here. I also tried to use NTPD but it was working for a day (enough to get offset pi-pc but failing to sync) and then it stopped. I also purged it because I have been told that ntpd has been deprecated on stretch.
I found the problem might be related to the pi not being able to communicate with the PC (which is however able to access the pi through VNC and ping it when directly connected through ethernet).
do not know why