Windows ISO 8601 timestamp

Get-Date supports Unix formatting strings with the -UFormat parameter. You can reuse it:

Get-Date (Get-Date).ToUniversalTime() -UFormat '+%Y-%m-%dT%H:%M:%S.000Z'

PowerShell's Get-Date supports standard .NET time formats. The o round-trip format complies with ISO 8601. Like so,

Get-Date -Format "o"

2017-08-15T12:10:34.4443084+03:00

Old question but since none of the other answers has it, if you're looking for UTC, this seems to do it:

(Get-Date).ToUniversalTime().ToString("o")

The following works both in Windows PowerShell 5.1 and PowerShell 7.0 on Windows/OS X:

 (Get-Date).ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ss.fffK")
 
 2020-12-01T22:31:41.402Z

If you don't want your milliseconds, then format string would be "yyyy-MM-ddTHH:mm:ss.000K"

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