You need to alias the subquery. Thus, your statement should be:
Select Z.id
From (
Select id, time
From dbo.tablea
Union All
Select id, time
From dbo.tableb
) As Z
Group By Z.id
You need to alias the subquery. Thus, your statement should be:
Select Z.id
From (
Select id, time
From dbo.tablea
Union All
Select id, time
From dbo.tableb
) As Z
Group By Z.id