Are you generating a new form each week?
If so, this will get the date for next Monday - you can play with the format:
var d = new Date();
d.setDate(d.getDate() + (1 + 7 - d.getDay()) % 7);
this.rawValue = d;
If you are using the same form for multiple weeks, then you would need to test to see if the current field value was less that the value generated by the script and replace in the field if needed.
Good Luck,