There isn't such a field in the JSON by default.
Bear in mind that the JSON file timestamp in S3, as well as the filename, do tell you when the AudienceStore file was written. This will not be the same as when each record in the file was written, because AudienceStore fills an internal buffer before writing the buffer to a file. Or else if 1 hour has passed since the last file was written, the buffer is flushed to file anyway. So depending on the amount of traffic to your AudienceStore connector, the file timestamp will be at most 1 hour older than all the records it contains. This may be close enough for what you are looking for here.
If not, then you'll need to check if there is already a suitable date attribute being included in the connector from the Visitor Profile. For example, if your AudienceStore connector was set up to send All Attribute at the end of a visit, then the built in date attribute "Last visit" would be suitable for you here, because that is set at the end of each visit, and this is the same time as the connector fires.
If the above does not apply, for example if you are not using an "All Attributes" connector action, you may still already be including in the connector a suitable date that is the same date/time as the connector fires. If not, you will likely need to set one up in AudienceStream and/or include it in the connector.
Thanks for that thorough response, Mark!
...then the built in date attribute "Last visit" would be suitable for you here, because that is set at the end of each visit, and this is the same time as the connector fires.
For consistency purposes, will this logic of using "Last visit" match the `updated` column found in the Redshift visitor data?
In the case I talked about, where you are using an AudienceStore connector at the end of the visit, it should be very close, but may not be identical.
You could instead change the built in Visitor Date Attribute "Last visit" to be enabled for AudienceDB, if it is not already.