Using a dummy word for dates makes all dates appear as the same term in the document-term matrix. After all, the main information of the document-term matrix is how many terms any pair of documents have in common and how specific each term is to any given document (see the TF-IDF measure). If each date was counted as a separate term, it would be quite possible that there is no overlap and that any subsequent algorithm to cluster or classify the documents will have difficulty recognizing the similarity between them. Also, many dimensionality reduction techniques identify topics be evaluating co-occurrences of terms. But if every date is interpreted as a different term, that makes it much harder to recognize co-occurrences of other terms with dates.