In the past, project models usually had a monthly period during construction, and a quarterly, semi-annual or annual period during operations. This made a lot of sense, because Excel allowed only a limited number of columns, and one didn’t want to run out of columns before the end of the project term!
However, this is not strictly necessary any more, since Excel now allows several thousand columns. Consequently, it is, in my opinion, far easier for the modeller to use the same period throughout the model. For a lenders’ model, I recommend that the period be one month per column. If you nonetheless want to use a multi-month period, you need to ensure that the number of months in the repayment period is divisible by the number of months in your model period.
Having a one-month period will mean that you may need to identify whether a non-monthly event occurs in a given period, such as a quarterly repayment. Modulus is a helpful function in this respect. E.g.
=if(mod(present month – starting month,3)=0,1,0)
could be used as a quarterly repayment indicator.
Period also has an effect on the way interest is calculated. If the interest compounding period is greater than that of the model, you will need to create additional rows to accrue uncompounded interest. More on this point in the Debt corkscrews article