Auto update item DateTime

Hi All,

My mqtt topic “home/rfm_gw/node1/START”, is updated only when the end node is started with some random 4 digit numbers. (eg:- 1123)

Number node1_START {mqtt="<[broker:home/rfm_gw/node1/START:state:default]"}
DateTime node1_UPTIME “Note Ready at [%1$tH:%1$tM]”

As soon as item node1_START is changed, node1_UPTIME should be set to current time.

Can someone tell me how to achieve this?