You don't need to solve this with SQL if you don't want to. In perl, TMTOWTDI. I would suggest you look at the module Date::Set. You should be able to add to the sets like this:

$a->event( start => '19971024T120000Z',
           end   => '19971025T120000Z' );  # one period
$a->event( start => '20001101T120000Z');   # open-ended
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Simple set-inclusion queries will tell you who is active on a given date. SQL might be better in the long run, but something like this would get you going.