1.open Registry Editor and navigate to this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Veeam\Veeam ONE Monitor\Service
2.Create a new DWORD value with the following parameters:
Value name = MaxTimeDiff
Value data (in DECIMAL format) = 43200
This will set maximum time difference timeout to 12 hours. According to your first post, that would be more than enough.
3.Restart Veeam ONE Monitor Server service and try to add a vCenter Server into Veeam ONE configuration one again.
Web Reference : https://www.veeam.com/kb1677