If both _time and yesterday are epoch you would need use a range i.e. _time>=yesterday AND _time<yesterday+86400 . Where yesterday should represent -1d@d or midnight.
If yesterday is string time, it should be of format YYYY-mm-dd and you would need to perform strftime(_time,"YYYY-mm-dd") to compare both dates as string time.
However, best option would be to fetch only the results from yesterday using base search filter i.e. either Time Picker or earliest=-1d@d latest=-0d@d-1s .
Please add more context for us to assist you with your issue, if one of the above does not fit the needs. Possibly ask a new question 😉
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