Problem in understanding datetime difference and invert - php

Question

I am having a problem in understanding the difference between datetime. I am using PHP datetime class.
Here is a small part of my code to calculate the datetime difference:
date_default_timezone_set("Asia/Kolkata");
function timeline_date($date)
{
$today = new DateTime();
$timeline_date = new DateTime($date);
$interval = $today->diff($timeline_date);
echo "<pre>";
print_r($today);
print_r($timeline_date);
print_r($interval);
echo "</pre>";
}
$date = "2019-02-02 04:36:02";
echo timeline_date($date);
What is the meaning of statement:
$interval = $today->diff($timeline_date);
Is it subtracting $timeline_date from $today?
Or is it subtracting $today from $timeline_date?
I am having a great confusion regarding this particular statement. I searched on Internet but nothing is clearly mentioned on it.
Also $timeline_date can be past or future date and even today's date. So does it matter we should subtract small quantity from large quantity?
I get following output from above code:
DateTime Object
(
[date] => 2019-02-05 16:11:37
[timezone_type] => 3
[timezone] => Asia/Kolkata
)
DateTime Object
(
[date] => 2019-02-02 04:36:02
[timezone_type] => 3
[timezone] => Asia/Kolkata
)
DateInterval Object
(
[y] => 0
[m] => 0
[d] => 3
[h] => 11
[i] => 35
[s] => 35
[weekday] => 0
[weekday_behavior] => 0
[first_last_day_of] => 0
[invert] => 1
[days] => 3
[special_type] => 0
[special_amount] => 0
[have_weekday_relative] => 0
[have_special_relative] => 0
)
We can see [invert] => 1 in above array. What is actual meaning of it? If I change the $date to:
$date = "2019-02-10 04:36:02";
Then [invert] => 0 changes to 1. Why? Does it mean if we have past date in difference then invert will be 0 and if we have future date then this array element will be 1?

Answer 1

DateTime::diff gives you the difference between the two dates, somewhat obviously. It doesn't really matter which one is bigger or smaller, a delta is a delta regardless of that. However, the resulting DateInterval object does note which one was bigger or smaller with the DateInterval::$invert property:
Is 1 if the interval represents a negative time period and 0 otherwise. See DateInterval::format().
The rule is if $b in $a->diff($b) is before $a, $invert is 1. Looking at said DateInterval::format, you'll see:
R Sign "-" when negative, "+" when positive
r Sign "-" when negative, empty when positive
So, if you do care about the future/past difference, you can use the %R/%r formatting parameters in DateInterval::format to output a "-" and/or "+". If you don't care, just omit that option and it's irrelevant.