function Get-DateISO8601() {
    [string]$date_string = ([System.DateTime]::Now | `
        Get-Date -Format yyyy-MM-ddTHH.mm.sszzz).ToString() | `
        foreach {$_ -replace ":", "."}
    return $date_string
}

Running the command Get-DateISO8601 returns the value

2019-01-06T12.17.57-08.00