How do I change the modified time of a file?

The DateLastModified property of the FileSystemObject is read-only. However, you can use Shell.Application to achieve this (thanks for the tip, Richard): 
 

<%      Sub Touch(FolderPath, FileName, NewDate)                    Set app = CreateObject("Shell.Application")          Set folder = app.NameSpace(FolderPath)          Set file = folder.ParseName(FileName)                    file.ModifyDate = NewDate            set file = nothing          set folder = nothing          set app = nothing      End Sub        Call Touch(Server.MapPath("/"), "somefile.htm", "2005-09-01")      Call Touch("C:\", "somefile.txt", "2012-01-01")  %>

 
I was unable to get this to work on my local system, but I have been assured by others that this will work. 
 
If it's a text-based file, you could read in the contents of the file, delete the file, and re-create it. That would alter the create *and* modify time of the file, and might not be the desired result (especially since you can't just pass in an arbitrary date/time). Here is the code to do this in VBScript: 
 

<%      Sub Touch(FolderPath, FileName)            set fso = CreateObject("Scripting.FileSystemObject")          set fs = fso.openTextFile FolderPath & FileName, 1, true          f = fs.readAll()          fs.close: set fs = nothing          fso.deleteFile FolderPath & FileName, true          set fs = fso.createTextFile FolderPath & FileName, true          fs.writeline(f)          fs.close: set fs = nothing          Set fso = nothing      End Sub        Call Touch(Server.MapPath("/"), "somefile.htm")      Call Touch("c:\", "somefile.txt")  %>

 
As a last resort, you could use Touch.EXE (from the Resource Kit), called via Shell.Application...