I just started as a BI Analyst and I am building my first dashboard.
I have been building an HR dashboard using ONE excel sheet containing data from the past 6 months (January until June) such as employer ID, name, position, gender age, hour worked, hire and termination date etc.
But now I am having troubles with figuring out how to implement new monthly data I will receive: in the future the plan is to use a folder with SEVERAL excel sheets containing the same data: 1 row (per employee) gets added at the bottom every month.
I want to be able to update my PBI each month where I can monthly extract of ALL the historical data (without duplicates) (e.g. YoY changes in headcount, etc.).
Basically have an evolution of the periodic monthly snapshot.
Can anyone help with this? I hope my question is somewhat clear.

I tried using a snapshot date for the files (the file that contains data from 2022 until July 2024, and one that contains data from August – snapshot dates: 31.07.2024, and 31.08.2024, , however I cannot filter for the months before 31.07.2024, even though my file contains information on that period.

Formula for headcount measure: 
Headcount = 
VAR CurrentDate = MAXX(Employees,Employees[Snapshot date])
RETURN 
CALCULATE(
    COUNTX(
        FILTER(
            Employees, 
            Employees[Contract start date] <= CurrentDate && 
            (ISBLANK(Employees[Contract end date]) || Employees[Contract end date] > CurrentDate)
        ),
        Employees[Employee ID]
)
+0)