When back, if I say "Out of office until Thursday" [duplicate]
Solution 1:
Until x means that the change occurs on/at x:
I am here until Friday. | From Friday I will not be here.
or
I am playing football until 7pm. | At 7pm I finish playing football.
or
The store is closed until March. | In March the store opens.
In your example:
I am out of the office until Thursday. | From Thursday I am back in the office.