Impact to an environment when NTP server is not available for 6-8 hours

[scalp]

I've a mgmt server managing 8 clusters of gws and all are in sync with a single NTP server. 

We have planned to upgrade the OS of the NTP server hence it'll be unavailable for a few hours say 6~8 hours. Wanted to know will there be any impact on the env ? 

Are there any best practices which we need to follow ? 

How long can mgmt and servers be in sync if the NTP server goes offiline ?

Or do we have to manually change the timings on each of the devices before doing the upgrade ? 

[Chris_Atkinson]

Have you considered configuring additional NTP servers for redundancy and are there any VPNs involved here?

[Timothy_Hall]

It is unlikely that the clocks will drift enough in a 6-8 hour period with NTP unavailable to affect anything, but some major areas to watch out for would be Intranet VPN tunnels using certificates for authentication in IKE Phase 1, ClusterXL sync, Management HA, and possibly SIC trust.  All these functions could be affected if the clocks get too far off between systems.