if you use the ma-now directive and combine it with a maMoment filter for your point values, you can then have it updating for the last 12 hours fixed.

<ma-now update-interval="1 SECONDS" output="timeNow"></ma-now> <!-- change to minutes or hours as required --> <ma-point-values point="device" values="seasonal" from="timeNow | maMoment : subtract : '12 hours'" to="timeNow" rollup="ACCUMULATOR" rollup-interval="1 DAYS"></ma-point-values>

I believe it's something to that effect off the top of my head...