I preplan what I will eat each day in my diary the night before. I wish there was a checkbox next to those items on the diary so that I could keep track of what I had or had not yet eaten.
Otherwise, when the time-of-day feature (https://forums.cronometer.com/discussion/933/time-of-day-early-access-beta) rolls out to all, it could be a marker of if and when it's consumed. On the beta the time is added via a single click on the website to a non-timestamped entry. I don't know how it would work on the phone app, though, because it's not available yet.
+1 I just sent an email to the support asking for this last week.
A timestamp could be a workaround in the meantime but definitely checkbox for "eaten" would be great.