ConvertTo-SapDate.ps1

#Requires -Version 5.1
<#
.SYNOPSIS
    Normalises m/d/y style dates into the yyyyMMdd form SAP fields expect.

.DESCRIPTION
    PowerShell port of src/vbs/DateFormatCustom.vbs. Accepts "/", "-" and "." as
    the separator, tolerates a trailing "h:m:s", and zero pads month and day.

    Unparseable input is a non terminating error, so a whole column can be piped
    through without one bad row killing the run.

.PARAMETER InputDate
    The date text. Accepts pipeline input.

.PARAMETER Separator
    Separator for the output. Default "" gives yyyyMMdd.

.EXAMPLE
    '6/21/2019 13:05:00' | .\ConvertTo-SapDate.ps1
    20190621

.EXAMPLE
    Import-Csv .\export.csv | ForEach-Object { $_.Date | .\ConvertTo-SapDate.ps1 -Separator '-' }
#>
[CmdletBinding()]
param(
    [Parameter(Mandatory, ValueFromPipeline)]
    [AllowEmptyString()]
    [string[]]$InputDate,

    [string]$Separator = ''
)

begin {
    Set-StrictMode -Version Latest

    # m/d/y is the order the AAE export produces; the year may be 2 or 4 digits.
    $formats = @(
        'M/d/yyyy', 'M/d/yy',
        'M-d-yyyy', 'M-d-yy',
        'M.d.yyyy', 'M.d.yy'
    )
}

process {
    foreach ($item in $InputDate) {
        $datePart = ($item -split '\s+', 2)[0].Trim()
        if ([string]::IsNullOrEmpty($datePart)) { continue }

        [datetime]$parsed = [datetime]::MinValue
        $ok = [datetime]::TryParseExact(
            $datePart,
            $formats,
            [Globalization.CultureInfo]::InvariantCulture,
            [Globalization.DateTimeStyles]::None,
            [ref]$parsed
        )

        if (-not $ok) {
            Write-Error "Not a recognised m/d/y date: '$item'"
            continue
        }

        $parsed.ToString("yyyy'$Separator'MM'$Separator'dd", [Globalization.CultureInfo]::InvariantCulture)
    }
}