The magic words REVISIONDAY, REVISIONMONTH and REVISIONYEAR are not resolved while previewing a page. REVISIONDAY is replaced with a 0, while month and year are an empty string. These keywords should contain the values of CURRENTDAY, MONTH and YEAR while previewing a page like it did in 1.17. Otherwise they should have the same, empty string when in preview.
When a page is saved, the magic words are replaced with the correct values. REVISIONUSER doesn't seem to be affected