RTC Calculated Value dependent on Status is triggered by any change

I have a calculated value script that remembers the most recent time that a work item has entered the "In Progress" state - it is triggered by Status (i.e. STATE).

It is working correctly when state is changed but the script is also being triggered whenever changes are saved whilst not changing state.

Any suggestions?

The script follows:

//Remembers when a work item enters the "In Progress" state most recently
dojo.provide("client.valueProvider.currentInProgressDate");
dojo.require("dojo.date.stamp");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function() {
    var WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
    dojo.declare("client.valueProvider.currentInProgressDate", null, {
        getValue: function(attribute, workItem, configuration) {
            var now = dojo.date.stamp.toISOString(new Date(), {milliseconds:true, zulu:true});
            var dayZero = dojo.date.stamp.toISOString(new Date(0), {milliseconds:true, zulu:true});
            var state = workItem.getValue(WorkItemAttributes.STATE);
            var myOldValue = workItem.getValue(attribute);
            if (state) {           
                switch (state) {
                    case 'taskworkflow.state.s2':                //In Progress
                        return now;
                        break;                
                    case 'taskworkflow.state.s1':                //New
                        if (myOldValue != null) {
                            return dayZero;
                        }
                        break;                
                }
            }
            return myOldValue;
        }
    });
}) ();