Balance sheet

The difference between as on & as at is 

As on - As on the date of the balance sheet (covers only day)

As at - As at the time of the balance sheet (covers both day & time of balance sheet)

The balance sheet presents a company's financial position at the end of a specified date. Some describe the balance sheet as a "snapshot" of the company's financial position at a point (a moment or an instant) in time. For example, the amounts reported on a balance sheet dated December 31, 2015 reflect that instant when all the transactions through December 31 have been recorded.

So as on means it suggests that transactions happened throughout the day - 31/03/2016

As at means it suggests that transactions happened at that point of time on such day - upto business hours of 31/03/2016

The balance sheet presents a company's financial position at the end of a specified date. Some describe the balance sheet as a "snapshot" of the company's financial position at a point (a moment or an instant) in time. For example, the amounts reported on a balance sheet dated December 31, 2015 reflect that instant when all the transactions through December 31 have been recorded.

So as on means it suggests that transactions happened throughout the day - 31/03/2016

As at means it suggests that transactions happened at that point of time on such day - upto business hours of 31/03/2016