Payments terms are defined this way: for example (today 15.12.2014) :

• Number of days = 90  and   Day of the month = 0  then ... payment date =  current date + 90  = 15.03.2015
• Number of days = 90  and   Day of the month = -1  then ... xxx = ( current date + 90 )  ...  payment date = last_day_of_month(xxx) = 31.03.2015
• Number of days = 90  and   Day of the month = 13  then ... xxx = ( current date + 90 )  ...  payment date = day 13 of next_month(xxx) = 13.04.2015

Day of the month - set -1 for the last day of the month. If it's positive, it gives the day of the next month. Set 0 for net days.

I can not comment on previous answer as I do not have 50 karma. So here goes my comment.

Thank you for your response.

Regulat days intervals work fine: 15, 30 or whatever number of days.

The way you describe the handling of the "Date of the month" is innovative from my previous beliefs. I will have to test different scenarios but somehow I feel it will not really help me. 

Of all the cases where I used -1, only with 90 days (starting on 15.12) did I get a perfect hit. I tried current month + 30, 45, 60 , 75 and 90. The responses should have been 30/1; 14/2; 1/3; 16/3 and 31/3. Instead I got 31/1; 31/1 (really off the mark); 28/2; 28/2 (another gorss miscalculaton) and, finally, yes, 31/3.