I know this is an old post but i was looking for the same info and found this post first, so just in case any one else ends up in the same boat I'm in. I was trying to pull the last time a server rebooted as well as the last time windows updates were installed. But i was running into the same UTC issue.
It does not appear that you can use the DateAdd function in SWQL.
In my case i was trying to convert from UTC to EST so i was able to use the AddHour function instead. (haven't tested if there are these functions for each time increment, I assume they are all there)
Here is my final code with the addition, see the bold text.
n.SysName as Host_Name,
AddHour(-4,n.LastBoot) as Last_Reboot ,
MAX(o.installdate) as Last_Update
from Orion.nodes as n
Left outer join
Orion.AssetInventory.OSUpdates as o
on n.nodeid = o.nodeid
where n.nodename like 'SERVER_NAME%'
Group By n.sysname, n.nodeid, n.lastboot
order by n.sysname ASC