this is already done in the web uploader. see the screenshot in this post: https://forum.inaturalist.org/t/loss-of-location-and-date-info/51639/3. it doesn’t look like there’s a similar missing date validation in the Android app, but i think you have go with a particular workflow to even get an observation without a date. i’m not sure about the iOS app.
what you’re describing here is possible by using the API, but it’s so close to the line – or even crosses the line – of machine-generated content that i would discourage you from going down that path.
if you wanted to add a comment as you review each observation, that’s possible and can be facilitated by various tools, as noted by dianastuder above and in various other comments in the forum (ex. https://forum.inaturalist.org/t/bulk-identification-tool/5029/36).
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that said,
i would add such warnings sparingly. for example, if a user uploads 5 observations with missing date, adding warnings per observation seems like overkill. instead, a single comment on one of the observations or a single direct message seems like it would be less annoying.