For now I don't care about timezones and just want to have a correct calculated value of how many days are between two dates. I think I'm pretty near to what I want, but for me there is always a full day missing.
That's what I got so far:
$start = new DateTimeImmutable('2018-07-31 12:30:00');
$end = new DateTimeImmutable('2018-08-28 00:00:00');
$diff = $start->diff($end);
$totalDays = (int) $diff->days; // 27
$hours = $diff->h; // 11
$minutes = $diff->i; // 30
$seconds = $diff->s; // 0
$hoursAsDays = $hours / 24; // 0.45833333333333
$minutesAsDays = $minutes / 60 / 24; // 0.020833333333333
$secondsAsDays = $seconds / 60 / 60 / 24; // 0
$result = $totalDays + $hoursAsDays + $minutesAsDays + $secondsAsDays;
var_dump($result); // 27.479166666667
For me there are 28 full days from august. I thought I would get something like: 28.45
EDIT: Users who add '2018-08-28' as end date will assume that it will count as a full day. How do I safely make this possible to be mentioned in my calculation?
You can do that simply with strtotime() PHP function:
$start = '2018-07-31 12:30:00';
$end = '2018-08-28 00:00:01';
echo $diffDays = (strtotime($end) - strtotime($start)) / 60 / 60 / 24;
Result:
27.479166666667
The result is true because there are exactly 27,5 days (in terms of 24 hours per day) between the two dateTime:
27 days and early a half day:
from 2018-07-31 12:30 to 2018-07-31 23.59 = +1/2 day tot: 0,5
from 2018-08-01 00:00 to 2018-08-01 23.59 = +1 day tot: 1,5
from 2018-08-02 00:00 to 2018-08-01 23.59 = +1 day tot: 2,5
from 2018-08-03 00:00 to 2018-08-01 23.59 = +1 day tot: 3,5
from 2018-08-04 00:00 to 2018-08-01 23.59 = +1 day tot: 4,5
from 2018-08-05 00:00 to 2018-08-01 23.59 = +1 day tot: 5,5
from 2018-08-06 00:00 to 2018-08-01 23.59 = +1 day tot: 6,5
from 2018-08-07 00:00 to 2018-08-01 23.59 = +1 day tot: 7,5
from 2018-08-08 00:00 to 2018-08-01 23.59 = +1 day tot: 8,5
from 2018-08-09 00:00 to 2018-08-01 23.59 = +1 day tot: 9,5
from 2018-08-10 00:00 to 2018-08-01 23.59 = +1 day tot: 10,5
from 2018-08-11 00:00 to 2018-08-01 23.59 = +1 day tot: 11,5
from 2018-08-12 00:00 to 2018-08-01 23.59 = +1 day tot: 12,5
from 2018-08-13 00:00 to 2018-08-01 23.59 = +1 day tot: 13,5
from 2018-08-14 00:00 to 2018-08-01 23.59 = +1 day tot: 14,5
from 2018-08-15 00:00 to 2018-08-01 23.59 = +1 day tot: 15,5
from 2018-08-16 00:00 to 2018-08-01 23.59 = +1 day tot: 16,5
from 2018-08-17 00:00 to 2018-08-01 23.59 = +1 day tot: 17,5
from 2018-08-18 00:00 to 2018-08-01 23.59 = +1 day tot: 18,5
from 2018-08-19 00:00 to 2018-08-01 23.59 = +1 day tot: 19,5
from 2018-08-20 00:00 to 2018-08-01 23.59 = +1 day tot: 20,5
from 2018-08-21 00:00 to 2018-08-01 23.59 = +1 day tot: 21,5
from 2018-08-22 00:00 to 2018-08-01 23.59 = +1 day tot: 22,5
from 2018-08-23 00:00 to 2018-08-01 23.59 = +1 day tot: 23,5
from 2018-08-24 00:00 to 2018-08-01 23.59 = +1 day tot: 24,5
from 2018-08-25 00:00 to 2018-08-01 23.59 = +1 day tot: 25,5
from 2018-08-26 00:00 to 2018-08-01 23.59 = +1 day tot: 26,5
from 2018-08-27 00:00 to 2018-08-01 23.59 = +1 day tot: 27,5
from 2018-08-28 00:00 to 2018-08-28 00:00 = +0 day tot: 27,5
With strtotime() you can convert a date/datetime string into instant version (count from epoch in seconds).
Regarding your edit, to consider the ending day in the calculation, set its time as 23:59:59:
$end = date('Y-m-d 23:59:59', strtotime('2018-08-28 00:00:01'));
Thus, the result will be:
28.479155092593