in below situation
To calculate a daily consumption based on a incrementing register. This is most commonly used for calculating daily kWh values
return p1.value - p1.ago(DAY)
somes, to avoid much resource execute one same time(et 59 59 23 * * ?),we setup the executionDelaySeconds,for example,executionDelaySeconds is 30 seconds,then we use
p1.value - p1.ago(DAY),
p1.ago(day) 's value is the last day 23:59:59 's value ,but the p1.value's time is not the firetime'value ,it is today 23:59:59+30seconds times value, it have Deviation.
so,if have some way to calculating exactly p1.value-p1.ago(DAY) in executionDelaySeconds case