INDEX MATCH SMALL likely function to find nth occurrences in a column

A and B3 and copy down:

Formula:

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A is a regular formula, B is an array one (committedd with Ctrl+Shift+Enter)
You may use in B3 unlimited range version,

Formula:

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but it will probably slow down calculations in your workbook, which is probably much more complicated than the piece you attached (I even cut it more for tests). I don't expect such range limiting would be needed for A formula, because in your version of Excel, the calculations algorithm of Excel shall limit the used range itself.

Note that because of removing duplicates by checking if any of earlier runs is already listed with MATCH, the formula always looks for the SMALL(… ,1), but MIN can't be used here, as it would return 0 as 4th date, while SMALL returns an error, which can be easily handled .