Add -FromUnixTime to Get-Date to allow Unix time input (#12179)

src/Microsoft.PowerShell.Commands.Utility/commands/utility/GetDateCommand.cs

@@ -40,6 +40,12 @@ namespace Microsoft.PowerShell.Commands
             }
         }
 
+        /// <summary>
+        /// Gets or sets whether to treat a numeric input as ticks, or unix time.
+        /// </summary>
+        [Parameter]
+        public SwitchParameter FromUnixTime;
+
         private DateTime _date;
         private bool _dateSpecified;
 
@@ -237,7 +243,14 @@ namespace Microsoft.PowerShell.Commands
             // use passed date object if specified
             if (_dateSpecified)
             {
-                dateToUse = Date;
+                if (FromUnixTime.IsPresent)
+                {
+                    dateToUse = DateTimeOffset.FromUnixTimeSeconds(Date.Ticks).UtcDateTime;
+                }
+                else
+                {
+                    dateToUse = Date;
+                }
             }
 
             // use passed year if specified

test/powershell/Modules/Microsoft.PowerShell.Utility/Get-Date.Tests.ps1

@@ -192,6 +192,15 @@ Describe "Get-Date" -Tags "CI" {
         $timeDifference.Milliseconds | Should -BeLessThan 1
         $timeDifference.Ticks        | Should -BeLessThan 10000
     }
+
+    It "-FromUnixTime works" {
+
+        # Test conversion of arbitrary date in Unix time: 2020-01-01​T00:00:00.000Z
+        Get-Date -Date 1577836800 -FromUnixTime | Should -Be (Get-Date -Date 637134336000000000 -AsUTC)
+
+        # Test converstion of Unix time start date: 1970-01-01​T00:00:00.000Z
+        Get-Date -Date 0 -FromUnixTime | Should -Be (Get-Date -Date 621355968000000000 -AsUTC)
+    }
 }
 
 Describe "Get-Date -UFormat tests" -Tags "CI" {