Don't use a calculated value. Use a rule. You'll have to modify the rule in the manifest to set the value for only the first one. You'll have to do the same when they enter data for it if you want to set a date for the second, third, etc..
The challenge is that you can't set a specific row in the Designer. You can by manually editing the manifest.xsf file but it's advanced. We cover this in our InfoPath Master class in June-July, and you should come to that! :)
Of course, the simpler way to do this is to use SetValue command in qRules but you can do it without code.
Email me or Hilary if you need help.