Change framerate for SRT subtitle files

Sync-SRT.ps1

function Sync-SRT {

    [CmdletBinding()]
    Param(
        [Parameter(Mandatory=$false)]
        [double]$SourceFPS,
        [Parameter(Mandatory=$false)]
        [double]$TargetFPS,
        [Parameter(Mandatory=$false)]
        [double]$milliseconds,
        [Parameter(Mandatory=$true)]
        $file
    )

    $file = get-item -literalpath $file
    $content = Get-Content -literalpath $file


    # Check which process is required
    if ($SourceFPS -and $TargetFPS) {
        # Process 1: $a and $b are required
        if (-not $SourceFPS -or -not $TargetFPS) {
            Write-Error "Parameters `$SourceFPS and `$TargetFPS are required for this process."
            return
        }
        for($i=0; $i -le $content.Length; ++$i)
        {
            if($content[$i] | Select-String -Pattern "^\d+:")
            {
                $timestart = $content[$i].Split(" --> ")[0]
                $timeend = $content[$i].Split(" --> ")[1]
    
                $timestart_timespan = ([datetime] $timestart).TimeOfDay
                $timeend_timespan = ([datetime] $timeend).TimeOfDay
    
                $diff = $SourceFPS / $TargetFPS
    
                $new_timestart = "{0:hh\:mm\:ss\.fff}" -f ($timestart_timespan * $diff)
                $new_timeend = "{0:hh\:mm\:ss\.fff}" -f ($timeend_timespan * $diff)
    
                $timestart = $new_timestart
                $timeend = $new_timeend
    
                $content[$i] = $timestart.ToString() + " --> " + $timeend.ToString()
                $content[$i] = $content[$i] -replace "\.",","
            }
        }
    
        $content | Set-Content (join-path $($file.DirectoryName) "$($file.basename).FIXED$($file.extension)")

    } elseif ($milliseconds) {
        # Process 2: Either $c or $d is required
        if (-not $milliseconds) {
            Write-Error "`$milliseconds is required for this process."
            return
        }
        for($i=0; $i -le $content.Length; ++$i)
        {
            if($content[$i] | Select-String -Pattern "^\d+:")
            {
                $timestart = $content[$i].Split(" --> ")[0]
                $timeend = $content[$i].Split(" --> ")[1]
    
                $timestart_timespan = ([datetime] $timestart).TimeOfDay
                $timeend_timespan = ([datetime] $timeend).TimeOfDay
    
                $new_timestart = "{0:hh\:mm\:ss\.fff}" -f ($timestart_timespan.add([System.TimeSpan]::FromMilliseconds($milliseconds)))
                $new_timeend = "{0:hh\:mm\:ss\.fff}" -f ($timeend_timespan.add([System.TimeSpan]::FromMilliseconds($milliseconds)))
    
                $timestart = $new_timestart
                $timeend = $new_timeend
    
                $content[$i] = $timestart.ToString() + " --> " + $timeend.ToString()
                $content[$i] = $content[$i] -replace "\.",","
            }
        }
    
        $content | Set-Content (join-path $($file.DirectoryName) "$($file.basename).FIXED$($file.extension)")

    } else {
        Write-Error "You must specify parameters for either Process 1 (`$SourceFPS and `$TargetFPS) or Process 2 (`$milliseconds)."
    }
}

            $new_timestart = "{0:hh\:mm\:ss\.fff}" -f ($timestart_timespan * $diff)
            $new_timeend = "{0:hh\:mm\:ss\.fff}" -f ($timeend_timespan * $diff)

            $timestart = $new_timestart
            $timeend = $new_timeend

            $content[$i] = $timestart.ToString() + " --> " + $timeend.ToString()
            $content[$i] = $content[$i] -replace "\.",","
        }
    }

    $content | Set-Content (join-path $($file.DirectoryName) "$($file.basename).FIXED$($file.extension)")
    
}