After much thought, I think I have found a solution.

1. I create a Powershell script to give me tomorrow's date. c:/tomorrow.ps1
    $a = (Get-Date).adddays(1)
    $b = Get-Date $a -DisplayHint Date;
    $b | Out-File -FilePath c:\tomorrow.txt

This way it creates a file with the date in txt.

2. Then I create a batch file (BAT extension) c:/tomorrow.bat
    ECHO OFF
    PowerShell.exe -Command "C:tomorrow.ps1"

3. I create a shortcut of the file tomorrow.bat in C:\Users\<user>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup

This way every time you start windows, the file will be updated with the desired date.

4. Then in 'The Bat!', when you create a template, you only have to paste:
%SUBJECT="The text you want - %PUT='c:/tomorrow.txt'"

If anyone can think of an easier way, please comment.
Thanks