Debtor & Creditor Days Calculation

Hi,all.

As someone relatively new to corporate banking, and with no accountancy qualification experience, how do I calculate Debtor and Credit Days for say month 1, month 2, month 3 etc. of a company's financial year? Obviously over a 12 month period this seems straight forward - e.g. Debtor Days / Sales x 365, but what calculation should I input to Excel for other periods. Taking 25% for 3 months seems too simple, and I wonder if I should be looking at a TTM period for more accuracy? Any guidance would be much apprectiated.