lock or disable a text box once it is populated

I am creating a form in InfoPath for work and within the form as the user is filling it out, the form itself will be doing real time calculations. I am fairly new at using InfoPath, and my problem is that once the user hits the start button on the form it sets a valve of "now" in another text box, but the user can hit start again and it resets this value. I would like to know if there is a way to lock out this text box so that hitting the start button again will not reset this time. I can not lock out the start button because it will be used for other functions while filling out the form