The “Ignore Date” option can be used to ignore the modification date of files in the directory comparison.
Without it, files whose dates differ are reported as different immediately — their contents are never read. That is fast, but it also means a file that was merely touched, copied or restored looks changed. Ignoring the date (usually together with Ignore Time) pushes the decision down to size and contents: pairs of equal size are then verified by their content hash, running in the background while you browse. The first such comparison takes longer because the files really are read; repeat runs are answered from the hash cache.
This is the setting for the question “are these two trees the same, regardless of when the files were written?” — comparing a copy against its original, or a build output against a reference.