HRA EXEMPTION FOR PART OF THE MONTH

In last financial year (FY2021-2022), my son shifted his job and moved to a rented house in another city.  He is getting HRA from his employer. He took a flat on rent from 13 March 2022. Before that he was staying with us.

While filing his ITR (old tax regime), can he claim HRA exemption for the period of 13 March 2022 to 31 March 2022?  

Is income tax rule permits HRA exemption for part of the month stay also or it should be whole month(s) only.

Thanks in advance for your valuable reply.