Bear in mind, that NTP servers are grouped under some names.
Each name may include number of NTP servers with different IPs.
There is an organization by name "Pool NTP" https://www.pool.ntp.org/
You can find here local or nearest NTP pools.
Some NTP servers or pools may be not active at the time you wish to make time correction, some may be busy at specific time.
So, it is recommended to keep in internal list number of NTP pools, resolve IP of less loaded NTP server , then use it to correct time.
If NTP return error (in most cases returned time is 0xFFFFFFFF as well), repeat request, then repeat IP resolution, and then replace NTP server name from list and repeat until you have good replay.
It is recommended