The ignoreOlderThan setting is based on the difference between the current time and the modification time on the file. So if the file is being appended to every day, then it's always going to show a time stamp of that day as the modification time and not be ruled out by the ignoreOlderThan setting.
It sounds like you more likely want something like the followTail option in inputs.conf, but don't leave it enabled for too long because the documentation implies that if you restart a forwarder leaving it enabled, the input may skip over events that were written to the file during the time it was restarting. If you start the forwarder with the followTail option enabled, wait a couple minutes, then take it out and restart, that should hopefully do what you are looking for.
