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• You could consider placeholder attributes in the input fields on your HTML to display the format of the data that should be entered.

• Your closing </style> tag isn't indented on the same level as the opening tag, it should be.

• Style-wise, It's easier on people if you can align all your <input>s to the same vertical values, rather than based on the length of the description.

• You have extraneous whitespace in some of your brackets: if ( finalRefund < 0.0 ), should be:
  if (finalRefund < 0.0)

• The naming behind outputPar could be improved; outputParagraph: for example.

• Rather than repeating outputPar.appendChild() over and over, move it to an array, and do it in a loop:

outputPar.appendChild(document.createTextNode("Product Purchase date: " + formatDate(productPurchaseDate))); outputPar.appendChild(document.createElement("br")); 

like:

var contentToBuild = [ "Product Purchase date: " + formatDate(productPurchaseDate), // More here ]; contentToBuild.forEach(function(element){ outputParagraph.appendChild(document.createTextNode(element)); outputParagraph.appendChild(document.createElement("br")); }); 

• Your usage of brackets around math operations is inconsistent:

var msPerDay = (1000 * 60 * 60 * 24); var termLeft = 1.0 - termUsed; 

I'd suggest sticking to the same usage throughout, the brackets seem clearer, but often aren't required.

• isValidDate() could use some re-arranging.

function isValidDate(date) { "use strict"; if ( Object.prototype.toString.call(date) !== "[object Date]" ) { return false; } else if ( isNaN(date.getTime()) ) { return false; } else { return true; } } 

Rather than testing isNaN(date.getTime()) second, test it first, and remove it from the if-else block.
Then, you can simply return the boolean value of Object.prototype.toString.call(date) !== "[object Date]".

into:

function isValidDate(date) { "use strict"; if (isNaN(date.getTime())) { return false; } return (Object.prototype.toString.call(date) === "[object Date]"); } 

Note that, in your code, you test against negatively (!==) the variable, and return false.
It'd be easier, in future, if you test against positively (===) and returned true.

• You could consider placeholder attributes in the input fields on your HTML to display the format of the data that should be entered.

• Your closing </style> tag isn't indented on the same level as the opening tag, it should be.

• Style-wise, It's easier on people if you can align all your <input>s to the same vertical values, rather than based on the length of the description.

• You have extraneous whitespace in some of your brackets: if ( finalRefund < 0.0 ), should be:

   if (finalRefund < 0.0) 

• The naming behind outputPar could be improved; outputParagraph: for example.

• Rather than repeating outputPar.appendChild() over and over, move it to an array, and do it in a loop:

   

   outputPar.appendChild(document.createTextNode("Product Purchase date: " + formatDate(productPurchaseDate))); outputPar.appendChild(document.createElement("br")); 

  like:

   var contentToBuild = [ "Product Purchase date: " + formatDate(productPurchaseDate), // More here ]; contentToBuild.forEach(function(element){ outputParagraph.appendChild(document.createTextNode(element)); outputParagraph.appendChild(document.createElement("br")); }); 

• Your usage of brackets around math operations is inconsistent:

   var msPerDay = (1000 * 60 * 60 * 24); var termLeft = 1.0 - termUsed; 

  I'd suggest sticking to the same usage throughout, the brackets seem clearer, but often aren't required.

• isValidDate() could use some re-arranging.

   function isValidDate(date) { "use strict"; if ( Object.prototype.toString.call(date) !== "[object Date]" ) { return false; } else if ( isNaN(date.getTime()) ) { return false; } else { return true; } } 

  Rather than testing isNaN(date.getTime()) second, test it first, and remove it from the if-else block.

  Then, you can simply return the boolean value of

   Object.prototype.toString.call(date) !== "[object Date]" 

  into:

   function isValidDate(date) { "use strict"; if (isNaN(date.getTime())) { return false; } return (Object.prototype.toString.call(date) === "[object Date]"); } 

  Note that, in your code, you test against negatively (!==) the variable, and return false. It'd be easier, in future, if you test against positively (===) and returned true.

contentToBuild = [ "Product Purchase date: " + formatDate(productPurchaseDate), // More here ]; contentToBuild.forEach(function(element){ outputParagraph.appendChild(document.createTextNode(element)); outputParagraph.appendChild(document.createElement("br")); }); 

var contentToBuild = [ "Product Purchase date: " + formatDate(productPurchaseDate), // More here ]; contentToBuild.forEach(function(element){ outputParagraph.appendChild(document.createTextNode(element)); outputParagraph.appendChild(document.createElement("br")); }); 

• You could consider placeholder attributes in the input fields on your HTML to display the format of the data that should be entered.

• Your closing </style> tag isn't indented on the same level as the opening tag, it should be.

• Style-wise, It's easier on people if you can align all your <input>s to the same vertical values, rather than based on the length of the description.

• You have extraneous whitespace in some of your brackets: if ( finalRefund < 0.0 ), should be:
  if (finalRefund < 0.0)

• parseFloat(daysElapsed / 365).toFixed(2): Note that leap years have 366 days in the year, and in a ten year cycle, like your example, you'd have at minimum, two years with extra days. Perhaps using a datetime function to calculate that would be better.

• The naming behind outputPar could be improved; outputParagraph: for example.

• Rather than repeating outputPar.appendChild() over and over, move it to an array, and do it in a loop:

• You could consider placeholder attributes in the input fields on your HTML to display the format of the data that should be entered.

• Your closing </style> tag isn't indented on the same level as the opening tag, it should be.

• Style-wise, It's easier on people if you can align all your <input>s to the same vertical values, rather than based on the length of the description.

• You have extraneous whitespace in some of your brackets: if ( finalRefund < 0.0 ), should be:
  if (finalRefund < 0.0)

• The naming behind outputPar could be improved; outputParagraph: for example.

• Rather than repeating outputPar.appendChild() over and over, move it to an array, and do it in a loop: