How can I give today's date as default value for a parameter in powershell

I'd like to create a script that helps to copy files modified in a time range. I would like to leave the $EndDate parameter as optional and in such-case, I'd like the script to use today's date as Default.

Below is the Script:

param (
  [Parameter(Mandatory=$True)]
  [string]$Path,

  [Parameter(Mandatory=$True)]
  [string]$targetDir,

  [Parameter(Mandatory=$True)]
  [string]$BeginDate,

  [Parameter(Mandatory=$False)]
  [string]$EndDate,

  [switch]$force
)

 Get-ChildItem -Path $Path -Recurse | Where-Object {$_.LastWriteTime -gt $BeginDate -and $_.LastWriteTime -lt $EndDate }| cp -Destination $targetDir -Force