The 7th Annual China PHP Conference

DateTime Arithmetic

The following examples show some pitfalls of DateTime arithmetic with regard to DST transitions and months having different numbers of days.

例1 DateTime::add/sub add intervals which cover elapsed time

Adding PT24H over a DST transition will appear to add 23/25 hours (for most timezones).

<?php
$dt 
= new DateTime("2015-11-01 00:00:00", new DateTimeZone("America/New_York"));
echo 
"Start: "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt->add(new DateInterval("PT3H"));
echo 
"End:   "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>

上の例の出力は以下となります。

Start: 2015-11-01 00:00:00 -04:00
End:   2015-11-01 02:00:00 -05:00

例2 DateTime::modify and strtotime increment or decrement individual component values

Adding +24 hours over a DST transition will add exactly 24 hours as seen in the date/time string (unless the start or end time is on a transition point).

<?php
$dt 
= new DateTime("2015-11-01 00:00:00", new DateTimeZone("America/New_York"));
echo 
"Start: "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt->modify("+24 hours");
echo 
"End:   "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>

上の例の出力は以下となります。

Start: 2015-11-01 00:00:00 -04:00
End:   2015-11-02 00:00:00 -05:00

例3 Adding or subtracting times can over- or underflow dates

Like where January 31st + 1 month will result in March 2nd (leap year) or 3rd (normal year).

<?php
echo "Normal year:\n"// February has 28 days
$dt = new DateTime("2015-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo 
"Start: "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt->modify("+1 month");
echo 
"End:   "$dt->format("Y-m-d H:i:s P"), PHP_EOL;

echo 
"Leap year:\n"// February has 29 days
$dt = new DateTime("2016-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo 
"Start: "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt->modify("+1 month");
echo 
"End:   "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>

上の例の出力は以下となります。

Normal year:
Start: 2015-01-31 00:00:00 -05:00
End:   2015-03-03 00:00:00 -05:00
Leap year:
Start: 2016-01-31 00:00:00 -05:00
End:   2016-03-02 00:00:00 -05:00

To get the last day of the next month (i.e. to prevent the overflow), the last day of format is available as of PHP 5.3.0.

<?php
echo "Normal year:\n"// February has 28 days
$dt = new DateTime("2015-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo 
"Start: "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt->modify("last day of next month");
echo 
"End:   "$dt->format("Y-m-d H:i:s P"), PHP_EOL;

echo 
"Leap year:\n"// February has 29 days
$dt = new DateTime("2016-01-31 00:00:00", new DateTimeZone("America/New_York"));
echo 
"Start: "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
$dt->modify("last day of next month");
echo 
"End:   "$dt->format("Y-m-d H:i:s P"), PHP_EOL;
?>

上の例の出力は以下となります。

Normal year:
Start: 2015-01-31 00:00:00 -05:00
End:   2015-02-28 00:00:00 -05:00
Leap year:
Start: 2016-01-31 00:00:00 -05:00
End:   2016-02-29 00:00:00 -05:00

add a note add a note

User Contributed Notes

There are no user contributed notes for this page.
To Top