Time differences between hosts

Hi to everyone;
I have an HP C700 Enclosure with ProLiant BL460c Gen10 and Gen9 Blade Servers;
All my hosts are with VMware ESXi, 6.5.0, 16576891,  all with NTP Server set and NTP service running.
I've seen that in a month the differences between host's time could be even 30 minutes as some hosts slow down their "heartbeat".
To resolve this, i run once a month a ps script to restart NTP service.
I try to find out some documents where I could find a solution but the search was unsuccesful.
Sorry for my english and thanks in advance.