Odd Date/Time format - powershell

Question

I have a single program in the Uninstalls registry key with an odd date/time. The value reported is '1533407816', but the actual date is most likely '20180804', which is what all the other Autodesk products installed that day report as.
I am using [datetime]::ParseExact($program.GetValue('InstallDate'),'yyyyMMdd',$null) to convert those dates to a proper DateTime, but not sure what mechanism I need for the odd one. Or if it's even meaningful data.

Answer 1

The value is likely a Unix time stamp, which is the number of seconds that have elapsed since midnight (UTC) of 1 Jan 1970.
You can convert this value to a [datetime] instance in local time as follows:
[DateTimeOffset]::FromUnixTimeSeconds(1533407816).LocalDateTime
To get a UTC [datetime] instance instead, use .UtcDateTime.
Alternatively, work directly with the [System.DateTimeOffset] instance returned by [DateTimeOffset]::FromUnixTimeSeconds().
Note:
Unix time stamps are signed 32-bit integers ([int]) in seconds.
As an aside, this means that the latest date that they can represent is in the year 2038, a limitation known as the Year 2038 problem.
By contrast, [datetime] instances have a resolution of 100 nanoseconds, called ticks, stored in the .Ticks property, whose type is [long] ([System.Int64] a signed 64-bit integer). To manually convert seconds to ticks, multiply them with 10000000L (1e7L).
Simple helper function:
function convertFrom-UnixTime {
param([int] $Value)
[DateTimeOffset]::FromUnixTimeSeconds($Value).LocalDateTime
}
In action, in the Eastern Time Zone, using the en-US culture:
PS> convertFrom-UnixTime 1533407816
Saturday, August 4, 2018 2:36:56 PM
Expressed in Coordinated Universal Time (UTC) (en-US culture):
PS> (convertFrom-UnixTime 1533407816).ToUniversalTime()
Saturday, August 4, 2018 6:36:56 PM

Answer 2

That odd value to me looks like a Unix Timestamp.
Here's a function that will convert it to a normal date
function ConvertFrom-UnixTimeStamp([Int64]$unixTimeStamp, [switch]$asUtc) {
if ($unixTimeStamp -gt 253402430399) {
Write-Error "Cannot convert $unixTimeStamp to DateTime. Only integer values up to 253402430399 are valid."
}
elseif ($unixTimeStamp -gt [Int32]::MaxValue) {
Write-Warning "The given value exceeds the [Int32]::MaxValue and therefore enters the Year2038 Unix bug.."
}
[DateTime]$origin = New-Object System.DateTime 1970, 1, 1, 0, 0, 0, 0, Utc
if ($asUtc) { return $origin.AddSeconds($unixTimeStamp) }
return ($origin.AddSeconds($unixTimeStamp)).ToLocalTime()
}
ConvertFrom-UnixTimeStamp 1533407816 -asUtc will yield a DateTime object:
Saturday, August 4, 2018 18:36:56