Unusual message from make

I recently bought a Synology NAS, set up the appropriate shared folders and then moved my Pi program source files from a Windows server to the NAS. I edited my source files from a Windows PC and then switched to a Pi to compile them using make. Then I received the following message.

Display of clock skew message

I checked the time on the Pi - that was correct and it was set up to use NTP, and its time zone was correct too. I then checked the Synology and found the time on that was set manually and it was running a few tens of seconds fast. After setting it to auto, the clock corrected itself and I was able to compile the program.

Synology date/time settings