Translation:He hasn't entered his office for a month.
May 16, 2020
The present tense is also possible but the passé composé better describes the repetition of a non-action all along the period of time, while the present describes the situation as it is today:
- Il n'est pas entré dans son bureau depuis un mois (he did not enter in his office during the past month)
- Il est absent de son bureau depuis un mois (he is still absent)