Calculation of 3 months notice period in indian it industry

Hi I am Vinny. I have recently resigned from my current IT company on 8th January,2013 and serving notice period of 3 months. They(HR and managers)denied me early release and asked to serve 3 months when contacted my HR about that as per 90days my notice period get over on 7th April,2013 to which she denied saying 90 days and 3 months are different and there fore i have to consider 8th april,2013 as last day. Hence i need to understand how does 3 months are calculated cause in that case also my 3 months completes on 7th april and not 8th april. Need help in this regard urgently so that i can clear things out else my joining would get delayed by 1 week because of one day. Thanks in advance.