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There are a lot of things that can catch you out when working with time zones, but PHP has you covered with built in Time Zone support for the DateTime class.
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Time zones can be a very difficult aspect
of web development to understand.
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There are different time zones all over
the world and
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most of these time zones are not as
logical as you would hope.
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Every country has a different time and
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you want to make sure that you're showing
the user in that country the right time.
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There are some tricky caveats.
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Not all time zones are exactly one hour
plus or minus UTC.
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So storing a plus or minus integer doesn't
work.
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Not all time zones have daylight savings
time.
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Political events can cause time zones to
change their off set.
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Many developers approach this by always
storing dates in their database’s UTC.
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Any date going in is converted to UTC on
input, and back from UTC on display.
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To calculate times and offsets, many
developers store plus 1 or
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minus 4 as an offset and put that in their
database as an integer.
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This means that if the country's offset is
minus four and
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a half hours, then the numeric
representation is minus 4.5.
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This is a decimal, and if you try to put
decimal numbers into an integer field,
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it'll be truncated by MySQL and only the
value 4 will be saved.
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Even storing a decimal number would not
help if DST or
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time zone change came into effect.
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The time being displayed to the user would
still be wrong by as much as an hour.
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The more robust way to work with dates and
times in PHP,
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is to store the actual time zone in the
database and not just the offset.
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PHP uses IANA time zones which are an
internationally recognized body in
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charge of handling mapping of these times
zones and keeping them up to date.
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Let's take a look at some code.
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First this might look like a lot of code,
but when broken down it's pretty simple.
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On line 3 we have a new date/time object
just as before, using a supported format.
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We've also specified the second argument
of the date time class which is
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an instance of date time zone.
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This accepts a string with a name of a
time zone, and
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UTC is the one we want here.
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Next we clone that date so we can keep a
copy of the original.
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We do this because date time is a mutable
class.
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That means the object will change state
when we run methods like set time zone.
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So that we can keep a copy we have to
fully clone the object.
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Then work with a brand new version.
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On line 7,
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we now use the set time zone method on our
new copy of the UTC date/time instance.
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And the string American New York is a
supported IANA time zone name.
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This will update the local date/time
object to have a new time zone and
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therefore a new time.
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Finally, at the bottom of the script,
we’re outputting two
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paragraphs with the date and time for UTC
and another for the time in New York.
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Let’s have a look at that in the preview.
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Great, it looks like that worked.
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1314 in UTC is now showing 0914 in New
York, which is exactly right.
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All dates should be stored as UTC in the
database.
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The date should always be displayed to the
user in their own time zone.
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This sounds complicated.
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But it simply means you need to convert
from UTC to the local time zone
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for output.
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Then convert from the local time zone to
UTC on input.
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Let's get back to our workspace and see
what that code might look like.
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This is basically the same piece of code
we had before.
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We just swapped things around.
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Now it's worth noting that you'll probably
take your input from a superglobal such
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as a post variable.
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But, for the sake of simplicity, we're
going to use just a raw variable here.
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Now, the rest of this is pretty much the
same.
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We're creating a local date/time from the
American New York timezone.
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We're cloning that to make a UTC date time
object, and
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then we're setting the time zone of that
to UTC.
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So now if we run this, it should look
exactly the same as before.
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Perfect.
We've still got 13 o'clock and
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we have nine o'clock right there.
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By storing the time zone name itself the
various offsets for
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day light savings time or [UNKNOWN]
changes are handled for you.
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All you then need to do is store all dates
in UTC and update the one output.
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So, just remember, all dates are stored in
a database as UTC.
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All users should have a time zone attached
to their own account.
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That time zone should be a valid IANA
name, and whenever you output dates,
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you need to set the time zone of that date
to match the user's time zone.
3:51
Let's try a few questions on UTC and time
zones.
3:54
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