The Ultima Online servers are constantly in the process of "backing up" the game. It takes about 45 to 90 minutes for each cycle to complete, though the length of time this takes depends greatly on the amount of data that is stored in each world.
The daily maintenance period will start at the first completed backup cycle after the maintenance window begins. For example, if all things were perfect, the backup would end at 5:00am, and the server would go down for maintenance at this time as well. However, if the server is in the middle of a backup at 5:00am, it will continue that backup to the end, and then go down for maintenance. Due to this factor of uncontrolled variability in the backup cycles, the downtime will not happen at exactly 5:00am each day.
Maintenance Schedule
Japanese Shards 8am JST:
Asuka
Hokuto
Izumo
Mizuho
Mugen
Sakura
Wakoku
Yamato
European Shards 5am GMT/UTC:
Drachenfels
Europa
Korean Shards 9am KST:
Arirang
Balhae
Taiwanese Shard 8am:
Formosa
Eastern USA Shards 5am EST/EDT
Atlantic
Catskills
Chesapeake
Great Lakes
Lake Superior
Legends
Siege Perilous
Western USA Shards 5am PST/PDT
Baja
Lake Austin
Napa Valley
Pacific
Sonoma
Origin
Australian Shard 5am AEST
Oceania
Veteran Rewards Database: Noon CST