Has anyone seen an interface that allows users to select two different sets of dates to compare data? And then the corresponding data in tables and/or graph form?
Even with just sets of integers (like [1 2 3] and [2 3 4 5]) asking what it means to "compare" the data is vague. Do you want to know which set has more numbers, what elements they have in common, which contains the largest or smallest number? Etc.
But if you just want a simple interface to compare two lists (kind of diff-like), you could sort them and show them such that if a date appeared in both sets it would be on the same row.
SET-1: [21-Jan-2001 08-May-1975 04-Aug-2000]
SET-2: [13-Mar-1987 04-Aug-2000 12-Apr-2001 08-May-1975]
SET-1 | SET-2 -------------+------------- 08-May-1975 | 08-May-1975 ... | 13-Mar-1987 04-Aug-2000 | 04-Aug-2000 21-Jan-2001 | ... ... | 12-Apr-2001
But the best idea is to go to your users/use-cases and find out what it is people are trying to find out from your data from this "comparison", and design according to that.