Maybe this will help:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US" lang="en-US">
<head>
<title>Medicine Supply</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-Script-Type" content="text/javascript" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<meta http-equiv="Content-Language" content="en-US" />
<meta name="Author" content="James Alarie - jalarie@umich.edu" />
<meta name="description" content="Calculate the day on which the medication supply runs out." />
<meta name="keywords" content="calculate,day,medication,supply" />
<script type="text/javascript">
/*<![CDATA[*/
function ComputeIt() {
f1=document.forms[0]; // abbreviation
I_StartDate =f1.F_StartDate.value;
I_AvailableTablets=f1.F_AvailableTablets.value;
I_NumberPerDayI =f1.F_NumberPerDay.selectedIndex;
I_NumberPerDay =f1.F_NumberPerDay.options[I_NumberPerDayI].value;
S_StartDate=new Date(I_StartDate)*1; // milliseconds since 1970-01-01
S_StartDate=S_StartDate/(24*60*60*1000); // ...days as a decimal
S_StartDate=Math.floor(S_StartDate); // ...days integer
S_Days=Math.floor(I_AvailableTablets/I_NumberPerDay);
S_OutOfSupplyAfter=S_StartDate*1+S_Days*1-1;
S_Date=new Date((S_OutOfSupplyAfter*1+1)*24*60*60*1000);
S_Year =S_Date.getFullYear();
S_Month=S_Date.getMonth()*1+1; // month as 1-12
S_Date =S_Date.getDate();
O_Date=S_Year+'-'+S_Month+'-'+S_Date;
f1.F_OutOfSupplyAfter.value=O_Date;
return true;
} // ComputeIt
/*]]>*/
</script>
</head>
<body class="body1">
<div id="body">
<!-- Page Header -->
<div id="header">
<h1>Medicine Supply</h1>
<hr />
</div>
<!-- Content -->
<div id="content">
<br />
<noscript>
<p class="notice">
You must have scripting enabled to make full use of this page.
</p>
</noscript>
<div class="center">
<form method="post" action="javascript:void(0);">
<div class="form">
<table border="1" cellspacing="2" cellpadding="2"
summary="This table is used for layout purposes only.">
<caption class="caption2"><b>Medication Supply</b></caption>
<tbody>
<tr>
<td>
<label for="F_StartDate">Start Date</label>
</td>
<td>
<input type="text" size="20" value="?" name="F_StartDate" id="F_StartDate" alt="First date" onfocus="this.select();" />
</td>
<td>
as YYYY-MM-DD
</td>
</tr>
<tr>
<td>
<label for="F_AvailableTablets">Available Tablets</label>
</td>
<td>
<input type="text" size="20" value="?" name="F_AvailableTablets" id="F_AvailableTablets" alt="Number of Available Tablets" onfocus="this.select();" />
</td>
<td>
Enter the number of tablets that you have
</td>
</tr>
<tr>
<td>
<label for="F_NumberPerDay">Number per Day</label>
</td>
<td>
<select name="F_NumberPerDay" id="F_NumberPerDay">
<option>1</option>
<option>2</option>
</select>
</td>
<td>
How many do you take each day?
</td>
</tr>
<tr>
<td colspan="3">
<input type="button" value="Compute" onclick="ComputeIt();" />
<input type="reset" value="Reset" title="Reset" />
</td>
</tr>
<tr>
<td>
<label for="F_OutOfSupplyAfter">Out of Supply After</label>
</td>
<td>
<input type="text" size="20" value="?" name="F_OutOfSupplyAfter" id="F_OutOfSupplyAfter" alt="Take the last Tablet on this Date" onfocus="this.select();" />
</td>
<td>
result: take the last tablet on this date
</td>
</tr>
</tbody>
</table>
</div><!-- form -->
</form>
</div><!-- center -->
</div>
<!-- Page Footer -->
<div id="footer">
<br clear="all" /><hr />
Written on December 23, 2012, by:
<a href="mailto:jalarie@umich.edu">James Alarie</a>.
</div>
</div>
</body>
</html>