Convert From Unix Time 
[timezone]::CurrentTimeZone.ToLocalTime(([datetime]'1/1/1970').AddSeconds($UnixTime))
Example
PS C:\> $UnixTime = '1553353149'
>> [timezone]::CurrentTimeZone.ToLocalTime(([datetime]'1/1/1970').AddSeconds($UnixTime))


Saturday, March 23, 2019 9:59:09 AM
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Convert To Unix Time 
[int][double]::Parse((Get-Date ($date).touniversaltime() -UFormat %s))
Example
PS C:\> $Date = Get-Date
>> [int][double]::Parse((Get-Date ($date).touniversaltime() -UFormat %s))

1553789149
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