The .at root nameservers reload times are the following:
06:00 am CET
10:00 am CET
01:00 pm CET
04:00 pm CET
07:00 pm CET
10:00 pm CET
A short example how it works: If you update the nameservers of your domain name lets say at 02:00 pm, these changes will immediately become visible in the whois record, but not become active until the root nameservers have been reloaded. The next reload would happen at 04:00 pm. After 04:00 pm the update would be replicated around the internet and your changes will become active.