Paul, you are saying that your internal NTP server (x450a-24x) is NOT receiving time from an external NTP source - then you should be fine, but you have to question your NTP setup in general. As you would have to manually adjust the time on your x450 every now and then.
If you do use external NTP servers, or plan to do so, you can also have a look at servers that implement the so called "Leap smear", where the 1 second is not inserted as extra second, but distributed over a time of 20 hours (that is, for 20 hours the clock is running slightly slower). Like Google does:
https://developers.google.com/time/smear
carsten