interest on loans

[murray Grey]

what is the formula for figuring interest on loans if the
loan time frame is for example: 5 months 18 days?
I can figure loan interest on monthly basis but what if
it involves an odd number of days, such as the
5 months and 18 days or say 10 months 14 days, etc.
Would this be figured as both a monthly rate and then
daily rate for the odd number of days? Considering that
all the months don't have the same amount of days.
What if the loan was for 10 months and 12 days at
4% interest ? Thank you for your help and assistance.
Leeann Springer