caculating dates with php - php

Question

I have a general question on calculating dates with php.
What happens if I store a timestamp like this in my database:
$db_timestamp = '2010-01-31 00:00:00';
and then run a daily script that checks if a month has passed since the timestamp was saved in the database:
if ($db_timestamp == make_unix_timestamp(mktime(0, 0, 0, date("m") - 1, date("d"), date("Y")), TRUE, 'eu')))
{
do something
};
my problem is that i just realized that this wouldn't work for all dates. in this case 'do something' would not be called in February, since February doesn't have a 31st day. any idea on how to implement something like that?

Answer 1

First, your DBMS should have a data type for date/time. They all store timestamps in a similar way.
MySQL then provides a function called UNIX_TIMESTAMP() if you need to return a timestamp PHP can understand.
SELECT UNIX_TIMESTAMP(`createTime`) FROM `articles`;
The opposite function is called FROM_UNIXTIME():
INSERT INTO `articles` (`createTime`) VALUES ( FROM_UNIXTIME(12345678) );
MySQL (or another DBMS for that matter, but I'm using MySQL as an example) has a slew of other functions to calculate time differences. For example, to know if an article is more than one month old, use can use DATE_SUB():
SELECT * FROM `articles`
WHERE `article`.`createTime` <= DATE_SUB(NOW(), INTERVAL 1 MONTH);
(In MySQL5 and above, you can also write it as such)
SELECT * FROM `articles`
WHERE `article`.`createTime` <= (NOW() - INTERVAL 1 MONTH);

Answer 2

$ts = strtotime($db_timestamp);
if ($ts < (time() - 2592000))
{
do something;
}
2592000 seconds = 30 days

Answer 3

You could use date_diff http://us3.php.net/manual/en/datetime.diff.php
or do a comparison of the timestamp in your database with
strtotime("-1 month");

Answer 4

You could check the timestamp using a query:
MySQL:
select date from table where date < now() - INTERVAL 1 MONTH;

Answer 5

It kind of depends on how you consider "one month".
If "one month" means "30 days", a solution would be to compare the timestamp you get from the database with the current timestamp :
$db_timestamp = strtotime('2010-01-31');
$current_timestamp = time();
var_dump( ($current_timestamp - $db_timestamp) / (24*3600) );
If the difference is 30 days... that's it.
A couple of notes :
strtotime converts a date to an UNIX timestamp-- i.e. the number of seconds since 1970-01-01
time returns the current UNIX timestamp
you can compare timestamps : they only represent a number of seconds ; and there are 24*60*60 seconds per day ;-)