I use Open Database quite a bit. JMP only loads data from the db when you issue the command, at which point it grabs the data an loads it into a table, independent of that database table. I'm confident there is no built in feature that will update the table whenever a db is updated.
You could perhaps write a JSL script to pull data from the db at specified intervals and compare them to the table, or just re-load the table altogether.
As for pulling data from a db and filtering for datetime data, it will depend on what type of database with which you are querying. For Transact SQL databases (Microsoft), you can do
SELECT FROM WHERE >= CONVERT(DATETIME, '2011-01-01 03:00:00', 102)