W2KSG: Date differences
The number of days between two dates has been covered before so we'll keep this one quick.
Script Center Home > Microsoft Windows 2000 Scripting Guide > Scripting Concepts and Technologies for System Administration > VBScript Primer > VBScript Reference > Working with Dates and Times Date Arithmetic
$test = [datetime]"10/10/2008"
$span = $test - $(Get-Date)
"Days until 10 October = $($span.days)"
Create a test date and then subtract today's date from it to create a timespan object. Display the number of days. Check out the other properties of timespan with get-member