i need to calculate month wise trainee days worked=

Column A-D is a data feed .

a) (Where the Trainee Start Date is before or equal to the final calendar day of the previous month AND the validated End Date is after the final calendar day of the previous month AND the validated End Date is before or equal to the final calendar day of the current month) If TRUE, divide the difference between the validated End Date and final calendar day of the previous month by the number of the days in the month

IF FALSE, move to the step below:

b) (Where the Trainee Start Date is before or equal to the final calendar day of the previous month AND the validated End Date is after the final calendar day of the current month) IF TRUE = 1

If FALSE, move to the step below:

c) (Where the Trainee Start Date is before or equal to the final calendar day of the current month AND the validated End Date is after the final calendar day of the current month) If TRUE, divide the difference between the final calendar day of the current month and the Trainee Start Date +1 by the number of the days in the month

If FALSE, move to step below:

d) (Where the Trainee Start Date is before or equal to the final calendar day of the current month AND the validated End Date is after the final calendar day of the previous month) If TRUE, divide the difference between the validated End Date and the Trainee Start Date +1 by the number of the days in the month.

Are you sure your formula is correct and returns the results on the image you shared with us ?

I’m wondering if the results column E are corrects.
Each time you have a condition on the “Start date” that needs to be before or equal to “31/12/2020” or “31/01/2021” for the column E. But for each start dates that are not in January 2021, the condition above will never be true… So I think that in Column E the cells will always be empty except for the first row.

Can you please verify your formula ore reformulate your problem if I did not understand it well ?
Or maybe you can provide the excel file so that we could see the formula ?

Apologies please ignore the formula above the below steps are the ones im trying to replicate in DAX.

WIP:
i just tried creating custom column for snapshot date between start date and end date and joined that to dim date which gives me the trainee count for each month but not the exact value of that month(if they ended in middle of month) i.e 0.5 rather than 1.