How have you concluded your NTP servers are actually reachable during those times?
No I have not checked that.
How to check, whether it is reachable or not?
Check if they are reachable with traceroute and ping.
To check the NTP server date & time you can use w32tm. Open a command prompt and run:
w32tm /stripchart /computer:IPorHostname /dataonly /samples:5