You're creating an island time zone. In effect you're creating a new Stratum 1 server for your network, and that can get tricky.
The fix for it is to use:
In your ntp.conf file. That tells NTP that your timezone is orphaned. But you need a new enough version (4.2.2) to use it. If you can't use, then use:
fudge 127.127.0.1 stratum 1
Also known as 'Undisciplined Local Clock'. Though I strongly recommend creating time pool, multiple servers who provide time mutually. It'll help slow down clock-drift.
And finally, if at all possible, using a GPS time-source is very well supported and will keep your time island close to true.