[EG] timestamp fix

[l0lawrence]

Fixes #31250

[openapi-pipeline-app[bot]]

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[openapi-pipeline-app[bot]]

Generated ApiView

Language	Package Name	ApiView Link
.Net	Azure.Messaging.EventGrid.SystemEvents	https://apiview.dev/Assemblies/Review/3e102a891e5445af8be8cde833dbb702?revisionId=dc070f946aac4398ac78fdf298bd2117
Python	azure-eventgrid	https://apiview.dev/Assemblies/Review/c554ddbabc6e40348998b1656e0ff585?revisionId=6302c44c81a54214a424aea9d0cf8e69
Swagger	Microsoft.EventGrid	https://apiview.dev/Assemblies/Review/8dab2e78889f4bf4a935473df37e62bf?revisionId=7fedf5cfd57041af97086275f25af0e3