Find Current Time and Subtract Previous Time in Seconds - php

Question

I'm trying to find the current date and time, convert to a unix timestamp, and then subtract a previous time. I've tried multiple approaches and have received errors or incorrect values. So far here is my code:
// Current date and time
$currentTime = date("Y-m-d H:i:s");
// Convert datetime to Unix timestamp
$currentTimestamp = strtotime($currentTime);
// Create previous date and time
$previousTime = new DateTime("2021-04-17 13:00:00");
// Specify display format
$previousTime->format('Y-m-d H:i:s');
// Convert to Unix timestamp
$previousTimestamp = strtotime($previousTime);
// Subtract previous time from current time
$time = $currentTimestamp - $previousTimestamp;
// Display result
echo $time;
So how it should work is if the current date and time is for example: 2021-04-17 14:00:00 and the previous date and time is 2021-04-17 13:00:00, then the result should be 3600. Or if there is a two hour gap, then it's 7200, etc. With this current code, the error I'm getting is:
Uncaught TypeError: strtotime(): Argument #1 ($datetime) must be of
type string, DateTime
And other code I have tried doesn't return the correct time difference or other errors are thrown. How do I get the correct time difference?

Answer 1

You need to read the docs about what each function expects as an argument and what each returns. You're mixing a timestamp (an integer) with a DateTime object. If you want to do date calculations, you'll need to use the same format for both. Since you're looking for the number of seconds difference, it's probably simpler to use timestamp integers.
This code gives you an integer timestamp:
$currentTime = date("Y-m-d H:i:s");
$currentTimestamp = strtotime($currentTime);
But note that "now" is the default return value for the time() function, so you can just do this instead:
$currentTimestamp = time();
And you don't need this:
// This gives you a DateTime object
$previousTime = new DateTime("2021-04-17 13:00:00");
// This doesn't change the internal representation,
// it just returns a value that you're not using.
$previousTime->format('Y-m-d H:i:s');
// This function expects a string, but you're giving an object.
$previousTimestamp = strtotime($previousTime);
Instead, you can just pass your formatted date string directly to strtotime() and it will return an integer timestamp:
$previousTimestamp = strtotime("2021-04-17 13:00:00");
Now you have two integers representing seconds, so you can just subtract them to get the number of seconds between. Your prog becomes:
$currentTimestamp = time();
$previousTimestamp = strtotime("2021-04-17 13:00:00");
$diff = $currentTimestamp - $previousTimestamp;
echo $diff;
Or just:
echo time() - strtotime("2021-04-17 13:00:00");