If statement to sum values based on date on two tables

Hello I am trying to create a measure to sum values with an if statement across dates.. At present I have three tables (see below for more details) - The dax I am trying to write would sum the value of subscribers based on date, if the date selected is greater that the highest date in the subscriber date, then sum the subscribers in the customers table

Thanks in advance

Ideally, it would look like this

Date:                         customer number            subscribers (the measure I am trying to make)

March 2019.              1                                      10 (Historic Value from Subscribers table)
April 2019                 1                                       12 (Historic Value from Subscribers Table)
May 2019                  1                                       13 (Current Value from Customer Table)

To sum the number historical subscriptions. I am using the following dax:
historical subs =
CALCULATE(
          SUM(Subscribers[subscribersCount]), 
          USERELATIONSHIP(Subscribers[date],'Date'[Date])
)

To get the sum of current subscriptions. I am using the following dax:
Current Subs =
         Sum(customers[subscribers)

I have tried adding both of these as variables then using an if statment:

If (date[date] > subscribers[date], current subs,historical subs)

Table: Subscribers (has historical subscriber numbers)
Columns:

• Date - Format (4/1/2019 all dates are at the first of the month)
• CustomerID
• Subscribers

Table: Customers (has current subscriber numbers)
Columns:

• CustomerID
• Customer Name
• Subscribers (current number)

Table: Date (has current subscriber numbers)
Columns:

• Date - Format: 8/1/2019
• Months - Format: April 2019