Apply a timeshift to an srt file and write to stdout.

srtshift.ps1

#!/usr/bin/env pwsh
param($srtFile, $shiftMiliseconds=0)

if (-not $srtFile) {
    throw "Must set srtFile path."
    return
}

$srtLines = Get-Content $srtFile

class Element {
    [int]$index
    [string]$span
    [string[]]$body = @()
    [string]ToString() {
        $sb = [System.Text.StringBuilder]::new()
        $sb.AppendLine($this.index.ToString()) | out-null
        $sb.AppendLine($this.span)
        $this.body | foreach { $sb.AppendLine($_) }
        $sb.AppendLine()
        return $sb.ToString()
    }
}

$elements = [System.Collections.Generic.List[object]]::new()
$element = $null

foreach ($l in $srtLines) {
    if ([string]::IsNullOrEmpty($l.Trim())) {
        if ($element) {
            $elements.Add($element)
            $element = $null
            continue
        }
    }

    if (-not $element) {
        $element = [Element]::new()
    }
    if ($l -match '^[0-9]+\s*$') {
        $element.index = [int]($Matches[0])
    }
    elseif ($l -match '^(?<first>[0]{2}:[0-9]{2}:)(?<firstSeconds>[0-9]{2},[0-9]{3}) --> (?<second>[0]{2}:[0-9]{2}:)(?<secondSeconds>[0-9]{2},[0-9]{3})$') {
        $firstSeconds = [int]$matches.firstSeconds.Replace(',','')
        $secondSeconds = [int]$matches.secondSeconds.Replace(',','')
        if ($shiftMiliseconds -ne 0) {
            $firstSeconds = $firstSeconds += $shiftMiliseconds
            $secondSeconds = $secondSeconds += $shiftMiliseconds
            $element.span = $Matches.first + $firstSeconds.ToString("N").Replace('.000','') + " --> " + $Matches.second + $secondSeconds.ToString("N").Replace('.000','')
        }
        else {
            $element.span = $Matches[0]
        }
    } elseif ($l) {
        $element.body += $l
    }
}

$elements | foreach { $_.ToString() }