What rate is the history being collected on Water Age Days? The script works in the script console and there is no reason it should not work in the gateway timer. Something seems amiss with the data, the only explanation is system.tag.queryTagHistory() is returning a blank dataset.
Unless there is something else writing to the tag which I find hard to believe if you created a new tag.
I see you have it working but for future reference, yes, any errors from gateway scripts go to the gateway log files in the status section of the gateway