Created this to help convert UNIX time to and from Windows time when interfacing with data from API's that output time in UNIX time.

ConvertTo-UnixTime.ps1

function ConvertTo-UnixTime {
    [OutputType([int64])]
    Param(
        # Date in UNIX time
        [Parameter(Mandatory,
        ValueFromPipeline=$true)]
        [DateTime]
        $DateTime
    )
    [Math]::Floor([decimal](Get-Date($DateTime).ToUniversalTime() -UFormat "%s"))
}

ConvertTo-WindowsTime.ps1

function ConvertTo-WindowsTime {
    [OutputType([DateTime])]
    Param(
        # Date in UNIX time
        [Parameter(Mandatory,
        ValueFromPipeline=$true)]
        [int64]
        $ticks
    )
    [DateTime]::new(1970, 1, 1, 0, 0, 0, 0, [System.DateTimeKind]::Utc).AddSeconds($ticks).ToLocalTime()
}