Can i allow an empty date field in a form? - php

Question

I have a Form with in php. I have it talking and inserting to the the MYsql database perfectly if i have all fields in the form filled in. Basically i have 3 text fields and 2 dates fields. i only want to chose either date field instead of both.
I have tried only inserting 1 date and it does not insert in to DB only when i have both dates chosen it works.
Below is the code which works when both date fields are chosen. i want to be able to only have 1 or the other or both if possible
<?php
require('db.php');
//Inserting Data Variables
if(isset($_POST['submit']))
{
echo $owner = $_POST['owner'];
echo $budget = $_POST['budget'];
echo $status = $_POST['status'];
echo $s_jan = $_POST['s_jan'];
echo $s_feb = $_POST['s_feb'];
//Query the database
$query = "insert into sbook(owner,budget,status,s_jan,s_feb)
values('$owner','$budget','$status','$s_jan','$s_feb')";
$insert_query = mysqli_query($con, $query);
}
?>
Here is the form
<form action="<?php echo $_SERVER["PHP_SELF"];?>" method="post">
<label>Owner</label>
<br />
<input class="input" type="text" name="owner">
<br />
<label>EST Budget:</label><br />
<input class="input" type="text" name="budget"/>
<br />
<label>Status:</label><br />
<input class="input" type="text" name="status"/>
<br />
<label>Jan</label><br />
<input class="input" type="date" name="s_jan"/>
<br />
<label>Feb</label><br />
<input class="input" type="date" name="s_feb"/>
<br />
<p>
<button name="submit" type="submit" class="btn btn-primary">Submit</button>
</div>
</div>
</form>
Above is the code which works when both date fields are chosen. i want to be able to only have 1 date or the other date or both if possible