Comparing Related Records from Previous Months

I'm trying to set up an automated way to avoid manually QCing records. I have parent Table A (matters) with child records in Table B (timecards). Lots of related timecards get created in the Timecard all the time, but once/month, we generate a specific type of timecard record (as in certain fields have specific values in them) for each matter, and I want to compare them to the previous month's version of that specific timecard. How would I go about doing that? Essentially I want to write a formula field that looks at the previous month's child record, determines if Field 1's value has increased or decreased by X%, and alert me if the value has changed by a certain threshold, either by setting the formula field's value to some phrase, or possibly triggering a subscription being sent to me with all of the parent matters that had children with values surpass a given threshold compared to the previous month.

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Harrison Smith
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