The count would start from the first period of the time frame whether its months or weeks.
Perfect! Thanks @ChrisHeathcote
To specify an end and start then you need to create a separate module where you state the start time period and the number of columns to be numbered.
Therefore, create a new module containing no dimensions and add two line items; Start period (Time period format ) and Number ( Number format )
Update the Count Reference to include a nested IF THEN ELSE.
=IF ITEM(Time) = NewModule.Start Period AND PREVIOUS(Previous Running Count) + 1<=NewModule.Number THEN PREVIOUS(Previous Running Count) + 1 ELSE 0
This should ensure that the running count only starts where the user specifies and only continues to the specified number of time periods into the future.