I’ll reply to myself as I believe I found the solution (and I’m a bit proud)…
// create a variable called day from the system "time"
var d = new Date();
d.setDate(d.getDate());
var year = d.getFullYear();
var month = d.getMonth()+1;
var day = d.getDate();
//create a variable from the sensor date called "sensorday"
var sensorday = flow.get('sensorday')
//subsctract both to see if = 0 (it's today) if not 0 (it's not today)
msg.payload = sensorday - day
return msg;
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