count rows in a period

Thanks so much Rocky! My Employee data is like below:

• Every employee only have 1 record
• Last Working Date is either blank or is their Last Working Date

And my Calendar table is as follows:

My question is:

1. I need to provide the headcount as of 31 Jan 2018. And the headcount criteria should be:
1. Date Join <= 31 Jan 2018 AND
2. Last Working Date >31 Jan 2018 OR Last Working Date is blank
2. I need to provide the no. of leavers in Jan 2018. And the criteria should be:
   1. Last Working Date >= 1 Jan 2018 AND
   2. Last Working Date <= 31 Jan 2018

I have searched for many sites and try to use the below formular but the result is not correct:

Active Employees =
CALCULATE(
 COUNTA(Employee[p_empno]), FILTER(Employee, (Employee[Date_Join] <= LASTDATE('Calendar'[DateKey]) && (Employee[Last_Working_Date]>= FIRSTDATE('Calendar'[DateKey]) || ISBLANK(Employee[Last_Working_Date])))))

This report will be provided each month with an updated database. Thus the Date will be changed from 31 Jan 2018 to 28 Feb 2018 next time. How could I manage the dashboard with changing the dates and then the data will be reflected correctly?