If you have DAY, MONTH and YEAR as numbers for a date, you can convert that to a date by using the following formula:
dateAdd(dateAdd(dateAdd(fromTimestamp(0), DAY - 2, "days"), MONTH - 1, "months"), YEAR - 1970, "years")
This uses the fact that fromTimestamp(0) gives you the date for January 1, 1970, and then adds the days, months and years to get to the target date.
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I attempted to create a property with a date formula to monitor dates in Notion. As I adjusted the date formats, the message "Type mismatch: ****** is not a date" continued popping up. It appears to be better now that I found one solution on Reddit.
Notes: I altered the original version to "DAY – 2" to ensure that it worked well in another formula.