Calculating hours as self employed?

[c1live]

I am currently self-employed as a dentist, how do I go about proving the number of hours worked?

My invoices from clients and contract agreement do not specify the hours worked.

I can provide rest of the documents such as Business incorporation, course transcripts etc.

I am longer in touch with the clients, so cannot get a reference letter from them.

Anyone applied as a dentist and can advise how they got around this issue?