Well, not all months are the same length, so it�s hard to say how long a month is.

However,

You can make a formula date field called

Original Start Date Plus 18 Months

with a formula of

AdjustMonth([Original Start Date], 18) will be 18 months after that start date.

So you could use that in a report filter.

If you really want a formula to calculate the difference in months, that is a more difficult formula.