[Identity] MSI doesn't work with Function Apps, expires on date format is incorrect.

[bterlson]

• Package Name: @azure/identity
• Package Version: 1.0.0-preview.2

Describe the bug
DefaultAzureCredential and ManagedIdentityCredential do not handle the expires on date format provided by MSI on Azure Function Apps (and possibly Azure App Service as well). It sends a date format like MM/DD/YYYY H:MM:SS AM/PM +MM:SS but expects a date format matching the regexp /(\d\d)\/(\d\d)\/(\d\d\d\d) (\d\d):(\d\d):(\d\d) (\+|-)(\d\d):(\d\d)/

Two possible fixes:

• Generalize the regexp matching.
• Just pass the date through to Date.parse which handles many date formats. Care would have to be taken to ensure Date.parse interprets these dates appropriately, but from what I can tell it's a pure superset of the current semantics.

[sophiajt]

#2 seems like the better path, though I think we would want to apply it and then watch for any possible side effects of the increase in lenience. Perhaps we could slate this for the preview 4 release?

[bterlson]

I feel like sooner is better, considering this is a core scenario for DefaultAzureCredential...

@schaabs do you have any thoughts on the excess leniency of Date.parse? Do we need a regexp?

[sophiajt]

Is #4631 also related to this issue?

[schaabs]

@jonathandturner, I believe the two issues are the same.