I want to delete a whole line in txt file where i found a string with DATE format.

Question

#The date have specific format  mm/dd/yyyy
#I Need to find a line where is date older than one year and then erase it
$Date = (Get-Date).AddDays(-365)
write-host "-----------------------"
$a= '\d{2}\/\d{2}\/\d{4}'
Select-String -pattern "$a"  -Path C:\test\file.txt -AllMatches |
    ForEach-Object {
        $Info = $_ | #
            Add-Member -MemberType NoteProperty -Name Date -Value $null -PassThru |     
            Add-Member -MemberType NoteProperty -Name Captured -Value $null -PassThru   
        foreach ($Match in $_.Matches) {
            try {
                $Date = [DateTime]::ParseExact($Match.Value,"MM\/dd\/yyyy",$null)     
             if($Datum -lt $Date){
                 write-host "$Date is up to date"
                    else{
                        write-host "$Date is old" }  # here should be command which delete the line
        catch {
                $Date = 'NotValid'

Answer 1

Or you can use Set-Content like in mjolinor's example, or just pipe to out-file like I suggested, but in both cases you'd need to enclose the get-content in parentheses:
$Date = (Get-Date).AddDays(-365)
$a= '\d{2}\/\d{2}\/\d{4}'
(Get-Content C:\test\file.txt) |
Where-Object {$_ -notmatch $a -or $(Select-String -pattern "$a" -inputobject $_ -AllMatches |
Foreach-Object {$good = $true;$(Foreach ($match in $_.matches) {If([datetime]$match.value -lt $date){$good = $False}};$good)})} |
Set-Content C:\test\file.txt
Make sure it works as expected then change the filename to file.txt or test it without piping it to file by reviewing it in the console first.  Do you have multiple dates on any lines of your text file?  If not, the other answers are more appropriate.
I hope this post has helped!