Strange Problem with a date in PHP

[davezeking]

Hi,

I've found a small problem with dates in PHP. I have a function which takes 3 arguments: a date, number of days and a boolean for past/future

e.g. If I want to find a day 7 days in the future from today I would say:

 add_days_to_date("2003-12-16", 7, true)

This function works fine for most days but I have noticed a problem when it's supplied with ONE specific date: 2003-10-20.

In the following example:

 add_days_to_date("2003-10-20", 7, true)

2003-10-26 is returned, only six days in the future!

It's the only date that does not correctly return seven days in the future that I have found so far.

My functions look like this:

function seconds_in_hour()
{
// Number of seconds in an hour
return 60 * 60;
}

function seconds_in_day()
{
// Number of seconds in a day
$hour = seconds_in_hour();
return $hour * 24;
}

function add_days_to_date($date, $days, $future)
{
// Returns a $date + $days in the
// future if true or in the past if false

$givenDate = mktime(0,0,0,substr($date, 5, 2),substr($date, 8, 2),substr($date, 0, 4));

$num_days = $days * seconds_in_day();

if($future)
$new_date = date("Y-m-d", $givenDate + $num_days);
else
$new_date = date("Y-m-d", $givenDate - $num_days);

return $new_date;

}

Basically I wondered if this is a bug in the PHP functions since it only happens on a specific date, or could it be something to do with UNIX time format?

I have version 4.3.0 of PHP running on IIS 5.1

I look forward to anyone being able to shed any light on this subject,

Regards,

Dave King.

[davezeking]

I found this in the manual shortly after posting the problem:

---

$tomorrow = mktime (0,0,0,date("m") ,date("d")+1,date("Y"));

Note:
This can be more reliable than simply adding or subtracting the number of seconds in a day or month to a timestamp because of daylight savings time.

---

Hmmmm....now when does daylight come into effect - the last weekend in October, around the 26-31 - looks like that's my problem,

I've reworked my function and it's now much smaller and more efficient.

Sorry for wasting your time - as always, it pays to read the manual!

Regards,

Dave.

[seby]

an even better method would be to use strtotime.

like:
date('m/d/y', strtotime('+1 day'));
date('m/d/y', strtotime('+2 days'));

you can do all kinds of things with this function, like convert dates to unix timestamp, add days, subtract, etc.

[davezeking]

Thanks for your reply, that's an interesting way of getting the date and one I wouldn't have thought of.

I'll be investigating this function further in the next few days as what I need to do is to find the next Monday after a specified date and then every following Monday for a set number of weeks.

From a quick skim of the documentation this function seems to offer this ability, though I'm not quite sure how yet!

Many Thanks,

Dave.