Summary of the new feature/enhancement

PR #12179 added FromUnixTime parameter to the Get-Date command.
In PowerShell 7.1.0-preview.3, the following command causes an error.

PS /> Get-Date -Date -1 -FromUnixTime
Get-Date: Cannot bind parameter 'Date'. Cannot convert value "-1" to type "System. DateTime". Error: "String '-1' was not recognized as a valid DateTime."

This command should support negative unix time.

On Ubuntu, the following commands will not return an error.

$ date --date "@-1"
Wed Dec 31 23:59:59 UTC 1969

Proposed technical implementation details (optional)

The Date parameter of the Get-Date command has type DateTime. If an integer value is passed for this parameter, it will be interpreted as Ticks. This must be a positive value.

In other words, the following code is invalid.

PS /> [datetime] -1
InvalidArgument: Cannot convert value "-1" to type "System.DateTime". Error: "Ticks must be between DateTime.MinValue.Ticks and DateTime.MaxValue.Ticks. (Parameter 'ticks')"

Unix time can take a negative value, meaning that it is before Unix Epoch.

To allow the Date parameter to take a negative integer value, the type of this parameter must be changed to long.
But this is a breaking change.

I think the FromUnixTime parameter should be able to take a value, as was suggested in #11719.