Month/quarter/semestre/year calculations

Hello everyone
Here's a tuff one, for me for sure

I have like a thousand records that shows me for each client the periode of invoice. That periode can be monthly, quarterly, semestre and anual.
The question is: Since he started the agreement on a particular month, would i send him an invoice on another month, depending on the periode.
For example: if a client like to receive an invoice 2 times a year (semestre) and he started on Mar-16, the next invoice will be on Sep-16. I have to ask, not only what kind of periode do we have, but also, is this month, the month that i have to send him an invoice.
Let me add an example file, maybe you can help me.
Many thanks for all your help
Best regards
Pedro

Olá Pedro
Considere the following table (in same sheet for example)
B22:B25 --> M A S T
C22:C25 --> 1 12 2 4
D22:D24 --> 1 1 6 3

Use the following formula:

Formula:

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PT:

Formula:

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and copy right and down
You can use as alternative

Formula:

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Pt:

Formula:

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See the file monthly_Calculations.xlsx