HRA calculation

Hi friends 

I have a query regarding calculation of HRA

for eg i left a company on 10th march then in my HRA calculation i shall consider rent paid for 10 months and 10days

secondly i joined a new company on 15th march then in this case should i consider rent paid for 15 days or I can consider rent paid for rest of 20 days  of march month???

Thanks & Regards

Kirti Sharma