Rule calculation of difference between two readings

I am maintaining an item with rain readings (total mm, incrementing at random times) - and would like to be able to calculate a “rate” as mm/hour.

I guess that I let an update of the “total mm” reading trigger a rule, which should then calculate the difference in time between now and the last update - and then difference in reading (mm) between them.
Dividing the mm amount towards the period (in hours) should give me the “rate”.

Can anyone suggest how this looks in a rule ? - it is primarily the “difference in time between now and the last update” I am struggling with?

This is what I use for calculating water consumption:

rule "Meter_update"
 	Item meterupd received update
 	val Number newMeter = meterupd.state as DecimalType
 	switch(updmeter.state as DecimalType) {
 		case 1: {
 			val oldMeter = wtrMeter.state as DecimalType
			val oldDate = new DateTime((wtrMeterDate.state as DateTimeType).calendar.timeInMillis)
			val newDate = new DateTime(now())
			val double daymsec= 86400000
			var double daydiff = (newDate.millis - oldDate.millis)/daymsec
			// if(daydiff==0) {daydiff=1}
			wtrMeterDate.postUpdate(new DateTimeType(newDate.toCalendar(null)))
 			logInfo("Meters","Updated water meter")

Works like a charm and is sort of similar to what you want to do.

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