Subtracting hours and minutes from input fields - php

Question

I have a form that has five input fields:
name="MultiRoomFromDate" //outputs the date in the following format "d-m-Y"
name="MultiRoomFromTimeH" // has a dropdown of "00" to "23"
name="MultiRoomFromTimeM" // has a dropdown of "00" to "59"
<select name="MultiFromDisplayTimeH" id="MultiFromDisplayTimeH" class="FormTime">
<option value="0">0</option>
<option value="3600">1</option>
<option value="7220">2</option>
<option value="10800">3</option>
<option value="14400">4</option>
</select>
<select name="MultiFromDisplayTimeM" id="MultiFromDisplayTimeM" class="FormTime">
<option value="0">0</option>
<option value="900">15</option>
<option value="1800">30</option>
<option value="2700">45</option>
</select>
If I select the "MultiRoomFromDate" as 26-01-2020" and the MultiRoomFromTimeH as "09" and the MultiRoomFromTimeM as "00" and the select the "MultiFromDisplayTimeH" as "01"and the "MultiFromDisplayTimeM" as "00" and submit the form the output is:
FromDate = MultiRoomFromDate; //26-1-2020
FromHours = MultiRoomFromTimeH; //09
FromMins = MultiRoomFromTimeM; //00
DisplayHours = MultiFromDisplayTimeH; //3600
DisplayMins = MultiFromDisplayTimeM; //0
What I need to do is output the "FromDate, FromHours, FromMins" MINUS the "DisplayHours" and "DisplayMins" as Y-m-d H:i.
This is what I have been working with:
$MultiRoomFromDate = trim($_POST['MultiRoomFromDate']);
$MultiFromDisplayTimeH = trim($_POST['MultiFromDisplayTimeH']);
$MultiFromDisplayTimeM = trim($_POST['MultiFromDisplayTimeM']);
$FromdateStamp = strtotime($MultiRoomFromDate);
$TodateStamp = strtotime($MultiRoomToDate );
$DisplayTime = $FromTimeStamp - $FromDisplayHours - $FromDisplayMins;
$DisplayDateTime = date("Y-m-d H:i", $DisplayTime);
$RoomFromTime = $MultiRoomFromTimeH.":".$MultiRoomFromTimeM;
if(!empty($MultiFromDisplayTimeH)) {
$DisplayTime1 = date('Y-m-d H:i:s', strtotime("-". $MultiFromDisplayTimeH." hours", strtotime($MultiRoomFromDate." ". $RoomFromTime)));
}
if(!empty($MultiFromDisplayTimeM)) {
$DisplayTime = date('Y-m-d H:i:s', strtotime("-". $MultiFromDisplayTimeM." minutes", strtotime($DisplayTime1)));
}
if(empty($MultiFromDisplayTimeH) && empty($MultiFromDisplayTimeM)){
$DisplayTime = date('Y-m-d H:i', strtotime($DisplayDateTime));
}
Also tried:
$t = $MultiFromDisplayTimeH + $MultiFromDisplayTimeM;
$RoomFromTimeStamp = strtotime($MultiRoomFromDate);
$h = $RoomFromTimeStamp - $t;
$DisplayTime = date("Y-m-d H:i:s",$h);

Answer 1

First, simplify your select option syntax by removing the value attributes -- it only makes your markup harder to comprehend at a glance and it will not be necessary for the datetime calculation that I will later demonstrate. I also think simpler field names would be beneficial.
<select name="subtractHours" class="FormTime">
<option>0</option>
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
</select>
<select name="subtractMinutes" class="FormTime">
<option>0</option>
<option>15</option>
<option>30</option>
<option>45</option>
</select>
Then, use PHP's datetime class to:
Initialize a datetime object from your date/time values
Subtract the submitted amount of time
Adjusted datetime string into the desired format.
Code: (Demo)
$fromDate = '26-1-2020';
$fromHours = '09';
$fromMins = '00';
$subtractHours = '1';
$subtractMins = '0';
$format = 'Y-m-d H:i:s';
$date = DateTime::createFromFormat('d-m-Y H:i:s', "{$fromDate} {$fromHours}:{$fromMins}:00");
$date->sub(new DateInterval("PT{$subtractHours}H{$subtractMins}M"));
echo $date->format('Y-m-d H:i');
Output:
2020-01-26 08:00
I added the :00 as seconds to the object for stability/consistency in case you wanted to insert the full datetime stamp into your database (which will require Y-m-d H:i:s format).
The above will process just as well when the time subtraction results in losing a day. (Demo)
$fromDate = '25-1-2020';
$fromHours = '01';
$fromMins = '30';
$subtractHours = '6'; // don't bother with 3600 precalculation
$subtractMins = '45';
// Output: 2020-01-24 18:45

Answer 2

I completely rewrote my code and I have now resolved the issue by using the following code:
$DisplayTime = $FromTimeStamp - $FromDisplayHours - $FromDisplayMins;
echo "DISPLAYTIME 1 ".$DisplayTime."<br/>";
$DisplayDateTime = date("Y-m-d H:i", $DisplayTime);