All this requires is one item to store the time stamp and a rule to update it whenever the sensor item receives a change. The only catch in my case is if the new value is the same as the current, it seems it is not triggered as an update. I am not sure if it is DomotiGa not sending or openHAB not triggering. So during the night I don't get as many updates. It's fine for now because when I setup a JeeNode to MQTT bridge, I can have that trigger the time stamp regardless if there is a sensor value change.
DateTime office_lastupdate "Last Update [%1$tm.%1$td.%1$tY %1$tr]"
You can use this site to help you format the datetime to suit your needs.
rule "Last Update Office" when Item office_lightlevel received update or Item office_temp received update or Item window_office received update then office_lastupdate.postUpdate(new DateTimeType()) end
Text item=office_lastupdate valuecolor=[office_lastupdate>300="red",>240="purple",>120="orange",>0="green",<0 data-blogger-escaped-code="" data-blogger-escaped-red="">
Using the value color option you can quickly at a glance look at the time since update based on the color. Just adjust the values in seconds.