$Culture = [System.Globalization.CultureInfo]::InvariantCulture
$Format = 'dd/MM/yyyy HH:mm:ss'
$Paths = "D:xxxyyyylog.txt","D:xxxzzzzlog.txt"
$Day = "20"

Foreach ($Path in $Paths)
{
  Get-Content $Path | where { [datetime]::ParseExact(([string]$_).Substring(0,19), $format, $culture) -ge (Get-Date).AddDays(-$Day) } | set-content $Path".new"
}

Get-Content $Path | if(!([string]$_)::IsNullOrEmpty) | where { ([datetime]::ParseExact(([string]$_).Substring(0,19), $format, $culture) -ge (Get-Date).AddDays(-$Day)) } | set-content $Path".new"