Day of the Year Program

So I have this code that creates a drop down box to select a month, another drop down box to select the day, then a text field to type in a year. My question is what to do next in php to be able to select a certain month, day, and year to where, when i click "calculate," it will display what day of that year it is?

So for example, if i choose January 4 2015, when i click calculate, i want it to display 4 since it is the 4th day of the calendar year. How do I do that?

<form action="calc.php" method="post">
Month: <select name="month">
<option value="january">January</option>
<option value="february">February</option>
<option value="march">March</option>
<option value="april">April</option>
<option value="may">May</option>
<option value="june">June</option>
<option value="july">july</option>
<option value="august">August</option>
<option value="september">September</option>
<option value="october">October</option>
<option value="november">November</option>
<option value="december">December</option>
</select>

Day: <select name="day">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
<option value="16">16</option>
<option value="17">17</option>
<option value="18">18</option>
<option value="19">19</option>
<option value="20">20</option>
<option value="21">21</option>
<option value="22">22</option>
<option value="23">23</option>
<option value="24">24</option>
<option value="25">25</option>
<option value="26">26</option>
<option value="27">27</option>
<option value="28">28</option>
<option value="29">29</option>
<option value="30">30</option>
<option value="31">31</option>
</select>

<input name="entry" type="text" size="10" placeholder="Year">

<input type="submit" value="Calculate">
</form>