DSO Day Sales Outstanding in Excel

Do you have a template or do you know an excel formula for calculating debtor days outstanding (financial DSO)?

I have found the below explanation on the net but I am not sure if the formula at the end is correct.

"If you average $30,000 a month in sales ($1,000 per day), and your A/R Balance is $30,000, then your DSO – A/R is $30,000 / $1,000 or 30 days.

If your A/R Balance is $60,000, then your DSO – A/R is $60,000 / $1,000 or 60 days.

When you think about it, it makes sense: if it takes 60 days on average to collect the money, then at the end of two months of selling $30,000 a month - from a standing start - you would have built up an Accounts Receivable

balance of $60,000. Each month after that, you would collect $30,000 from the 2nd prior month, and make another $30,000 in sales, so that your Accounts Receivable balance would stay at a constant $60,000.

Of course, neither Sales nor Accounts Receivable stay constant. The trick is to choose a time period over which to calculate Daily Sales. One month is too short, and a year too long – I favor a three month time period. Then you can calculate the DSO – A/R each month by taking the A/R Balance and dividing by the last three months worth of Sales, converted to a Daily Sales rate by dividing by 90.

Excel
Assume monthly values are in columns A, B, C, etc. You can calculate DSO – A/R starting in column C:

Sales
Accounts Receivable
DSO A/R in column C = C2/((A1+B1+C1)/90) in column D = D2/((B1+C1+D1)/90)"