ConvertFrom-EpochTime.ps1

function ConvertFrom-EpochTime
{
    param
    (
        [Parameter(Mandatory = $true)]
        [Double]
        $EpochTime
    )

    $epochstart = Get-Date -Date 1/1/1970
    Write-Output ($epochstart.AddSeconds($EpochTime))
}

ConvertTo-EpochTime.ps1

function ConvertTo-EpochTime
{
    param
    (
        [Parameter()]
        [DateTime]
        $Timestamp
    )

    if(-not $PSBoundParameters.ContainsKey('Timestamp'))
    {
        $Timestamp = Get-Date
    }

    $epochstart = Get-Date -Date 1/1/1970
    
    Write-Output (($Timestamp - $epochstart).TotalSeconds)
}

Epoch Time Converter

Two scripts to convert to and from Epoch Time.
Epoch Time is the number of seconds since January 1st 1970.