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To determine the number of unique dates in a SQL database, you can use the DISTINCT keyword with the DATE function. Here is an example query:

SELECT COUNT(DISTINCT DATE(datecolumn)) FROM tablename;

This query counts the number of distinct dates in the "datecolumn" of "tablename". The DATE function extracts the date portion of the datetime value in the column. The DISTINCT keyword ensures that each unique date is counted only once. The result of this query will be the number of unique dates in the table.