X31 is configured as a NTP server in the ntp.org global pool. As a result of this is serves thousands of machines the exact time. You can see the current performance statistics here: http://www.ntppool.org/scores/81.171.44.131
If you want to use X31.com as your time server you can explicitly select ams1.x31.com as your NTP time server. However I recommend you use a set of pool servers which you can find here: http://www.ntppool.org/en/use.html
Why does X31 provide a public time server? Its simple, X31 has gained a lot from free software and services over the years, its good to give something back and the load on the link is only about 50Kbit.