Why does date gives me a wrong date? - php

Question

I want to calculate a date based on a timestamp and some other informations.
My function looks like:
function getLastDeliveryDate($timestamp,$endOfMonth=true,$extraMonth=0){
$days = 0;
$extraDays = 0;
$endOfCurrentMonth = 0;
$tsDay = 86400;
if($endOfMonth){
$endOfCurrentMonth = date("t", $timestamp) - date("d",$timestamp);
//rest of days in current month. In this sample 16 days
}
for($i=0;$i<$extraMonth;$i++){
$x = $i + 1;
$date = new DateTime(date("Y-m-d", $timestamp)); //create dateobject to add a month
$date->modify("+{$x} month"); // add the month (next month)
$extraDays += date("t", strtotime($date->format("Y-m-d")));
// get the days of the selected month and add them to count
// in this case its 31 + 30 + 31 = 92
}
$days = $endOfCurrentMonth + $extraDays;
// count everything together 16 + 92 = 108 days
return date("d.m.y", $timestamp + ($tsDay*$days));
//returning date with 108 days added.
}
As a sample I call the function like:
// the timestamp is 2015-07-15
echo getLastDeliveryDate(1436911200, true, 3);
// should return 2015-10-31
But this return 2015-10-30 and I don't know why. But 108 Days shold be 2015-10-31. Whats going wrong here ?
If I call
echo getLastDeliveryDate(1436911200, true, 2);
Its correct and gives me 2015-09-30
Actually I allways want the last day of the month.
EDIT:
Wired, if I test this here: IDEONE everything works fine. Im my Project it doesn't :(

Answer 1

You need to create the datetime object before the loop:
$date = new DateTime(date("Y-m-d", $timestamp)); //create dateobject to add month
// simpler alternative: $date = new DateTime("#$timestamp");
for($i=0;$i<$extraMonth;$i++){
$date->modify("+1 month"); // add the month (next month)
// $extraDays += date("t", strtotime($date->format("Y-m-d")));
// you can reduce this line to:
$extraDays += $date->format("t");
}
// Result: 15-10-31
otherwise there is always 31 added because you use the day of the timestamp + 1 month.
Note:
You can reduce the whole function to this:
function getLastDeliveryDate($timestamp,$endOfMonth=true,$extraMonth=0){
$date = new DateTime("#$timestamp");
$date->modify("+$extraMonth month");
if ($endOfMonth)
$date->modify("last day of this month");
return $date->format("d.m.y");
}

Answer 2

The problem is the daylight savings time. You loose one hour on the 25th of october 2015. Since your timestamp is exactly 0:00:00 you lose one hour resulting in "30.10.2015 23:00:00" what should actually be 0:00:00
function getLastDeliveryDate($timestamp,$endOfMonth=true,$extraMonth=0){
$days = 0;
$extraDays = 0;
$endOfCurrentMonth = 0;
$tag = 86400;
if(date( 'H',$timestamp)==0){$timestamp+=3601;}
if($endOfMonth){
$endOfCurrentMonth = date("t", $timestamp) - date("d",$timestamp);
}
$date = new DateTime(date("Y-m-d", $timestamp));
for($i=0;$i<$extraMonth;$i++){
$date->modify("+1 month");
$extraDays += $date->format("t");
}
$days = $endOfCurrentMonth + $extraDays;
return date("d.m.y", $timestamp + ($tag*$days));
}
echo getLastDeliveryDate(1436911200, true, 3);
This code has a dirty fix for this problem by adding one hour and one second if your datetime is fixed to 0:00:00. When you don't care about the hours themselves, then this solution will fix your problem and is viable in any case. If you care about the hours, you have to check whether you are in daylight savings time or not and act acordingly.