Workaround 1: Export to a different format other than GeoJSON. For example, testing shows that this date value conversion issue does not occur when exporting a hosted feature layer to CSV.
Workaround 2: Post-process the exported GeoJSON to convert the big integer values back into acceptable date values.
Additional information:
- Note that neither GeoJSON nor JSON specify how dates should be represented.
- Note that JavaScript does specify how dates must be formatted in JSON, as indicated by the Date object’s toJSON method: 2012-04-23T18:25:43.511Z
- Whatever format dates are converted into during Export to GeoJSON should be human-readable, recognizable, and able to be re-converted back into Date values when re-imported into ArcGIS Online.