most of the companies follow the method 2.5 days per month, in other words it is earned every month ..also known as Earned leave.
The calculation purposes & for tax – accounts reasons – it is taken for the period Apr- Mar(financial year). Hence 30 days accrue to the employees on 1st Apr after working from Apr to 31st Mar.
The policies of leaves vary from company to company, just for the case above as you mentioned some companies take only working days -5 days leave only.
Some company esp in our case we allow only prefix or suffix only but not both viz if one takes leave on Fri (sat & sun weekly off) then one day leave & joins on Tues then only three days are counted as leave.
Its all depends upon the culture & management views.