add utc to osx date

The command

end=`date -v+30M '+%Y-%m-%dT%H:%MZ'`

for OSX should be

end=`date -uv+30M '+%Y-%m-%dT%H:%MZ'`

or the timezone won't be utc.

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#### Document Details

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* ID: 92115d19-d7a0-93a8-bf2b-5126d1cddcd0
* Version Independent ID: a657f09b-e1ac-12a2-4b9c-7aaa1f0e3f5c
* Content: [az storage account](https://docs.microsoft.com/en-us/cli/azure/storage/account?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/storage/account.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/main/latest/docs-ref-autogen/storage/account.yml)
* Service: **storage**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**