Hi Sriram,
Calculation should be , if you have maintain 12 days per annum in your base entitlement,
Pro rate basis -- 12 days of entitlement divided by 12 months that means one day per month.
Daily basis , 12 days of entitlement divided by 365 days
No pro rate basis - 12 days of entitlement will be added at every month.
for example , 01-01-2013 the entitlement is 1 day (as per above calculation)
30-04-2013 the entitlement is 4 days.
Thanks
Siva