help with sum of previous periods

[Wirt]

hello,

please tell me how to calculate the the sum of previous periods?

suppose that

Jan.2014 = sum 25 months ago
Fab.2014 = sum 28 months ago
Mar.2014 = sum 26 months ago

I mean that the number of months may be different.
who knows tell me please
thank you

[Alan Kirk]

please tell me how to calculate the the sum of previous periods?

suppose that

Jan.2014 = sum 25 months ago
Fab.2014 = sum 28 months ago
Mar.2014 = sum 26 months ago

I mean that the number of months may be different.

Personally I haven't had a particularly "Fab".2014 to date, but that aside... "different", based on what? As per the Request For Assistance Guidelines, (3) Try to make the question as specific as possible. ), let's dump the (increasingly annoying[1]) hypotheticals and describe exactly what it is that you're trying to do, and by what method. How is the dimension structured? What input(s) drive the number of periods that you're after? Anything else that may be relevant?

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[1] From a number of recent threads, I know that I'm not speaking for myself alone in saying this.

[Wirt]

Ok, I will try to explain in more detail
we have the following picture:

.......................Jan.14|Fab.14|Mar.14|...
Sum of prev.per...|.......| .......| .......|
Number of months.|..25..|..28...|...23..|...

(sorry for such a view, I'm the first on the site)

Number of months - calculated by the formula, and we get some value (25, 28, 23 ... )

Sum of prev.per - looks at values ​​in the cell "Number of months", and calculates the sum of months ago

for example:
Sum of prev.per(Jan.14) = sum 25 months ago
Sum of prev.per(Fab.14) = sum 28 months ago
Sum of prev.per(Mar.14) = sum 23 months ago ...
and so on

if you need more details, write, I will answer
thank you.