SetSharePointTimeZone.ps1

if($null -eq $adminCred)
{
    $adminCred = Get-Credential -UserName "" -Message "Gloabl SharePoint Admin Login for Office 365"
}
if($null -eq $tenantName)
{
    $tenantName = Read-Host "Please Enter your TenantName"
}
if($null -eq $timeZoneId)
{
    $timeZoneId = 4 # German Timezone
}

Connect-PnPOnline "https://$($tenantName).sharepoint.com" -Credentials $adminCred
$sites = Get-PnPTenantSite
Disconnect-PnPOnline


Connect-PnPOnline "https://$($tenantName)-admin.sharepoint.com" -Credentials $adminCred
foreach($site in $sites)
{
    Write-Host "loading $($site.Url)" -ForegroundColor Green
    Set-PnPTenantSite -Url $site.Url -NoScriptSite:$false -ErrorAction SilentlyContinue
}
Disconnect-PnPOnline

foreach ($site in $sites)
{
    Write-Host "loading $($site.Url)" -ForegroundColor Green

    Connect-PnPOnline $site.Url -Credentials $adminCred
    $web = Get-PnPWeb -Includes RegionalSettings,RegionalSettings.TimeZone,RegionalSettings.TimeZones
    
    $customTimeZone = $web.RegionalSettings.TimeZones | Where-Object -FilterScript {$_.Id -eq $timeZoneId}

    if($web.RegionalSettings.TimeZone.Id -ne 4)
    {
        $web.RegionalSettings.TimeZone = $customTimeZone
        $web.Update()
        $web.Context.ExecuteQuery()
    }

    Disconnect-PnPOnline
    Write-Host "done $($site.Url)" -ForegroundColor Yellow
}

Connect-PnPOnline https://$($tenantName)-admin.sharepoint.com -Credentials $adminCred
foreach($site in $sites)
{
    Write-Host "loading $($site.Url)" -ForegroundColor Green
    Set-PnPTenantSite -Url $site.Url -NoScriptSite -ErrorAction SilentlyContinue 
}
Disconnect-PnPOnline