Finding midnight of a timezone given offset in minutes [PHP] - php

Question

I have an offset from UTC stored in minutes: e.g -240
I'm trying to find the corresponding UNIX timestamp of midnight of the current day for this particular offset.
I found similar information in questions like this one: How do I get the UTC time of "midnight" for a given timezone?
However, I don't have the city name/timezone jurisdiction, just a minute offset. I think this should be fine since for my purposes I don't need to account for daylight savings, it can be off by an hour and still be fine.
Examples
Offset: -420
Midnight on 7/12/2014: 1405148400 (unix TS)
With UTC, I would have to first tell if it's the next day or same day as the TZ because it may have a different "last midnight".

Answer 1

While this solution looks a little ugly it does do what I think you're asking for! This example uses -180 minutes as the offset.
date_default_timezone_set('UTC');
// Work out which day the time zone is in
$day = strtotime('-180 minutes');
// Strip of the time part of the day, to give UTC midnight on the correct day
$utcMidnight = strtotime('midnight', $day);
// Now apply the offset in reverse to give the zone's midnight
$zoneMidnight = strtotime('+180 minutes', $utcMidnight);

Answer 2

You could use date_default_timezone_set to make all time-related functions acknowledge the shift. First thing to do is to convert those minutes into hours, since the UTC gap is 1 hour between n and n+1.
$hours = $minutes / 60;
I would also recommend that you check the minutes values first :
if($minutes % 60 == 0) // We're good.
Now, if you want to convert the UTC offset to a timezone, you can create your function :
<?php
function offsetToTimezone($offset){
$timezones = array(
"-12" => "Pacific/Kwajalein",
"-11" => "Pacific/Samoa",
"-10" => "Pacific/Honolulu",
"-9" => "America/Juneau",
"-8" => "America/Los_Angeles",
"-7" => "America/Denver",
"-6" => "America/Mexico_City",
"-5" => "America/New_York",
"-4" => "America/Caracas",
"-3.5" => "America/St_Johns",
"-3" => "America/Argentina/Buenos_Aires",
"-2" => "Atlantic/Azores",
"-1" => "Atlantic/Azores",
"0" => "Europe/London",
"1" => "Europe/Paris",
"2" => "Europe/Helsinki",
"3" => "Europe/Moscow",
"3.5" => "Asia/Tehran",
"4" => "Asia/Baku",
"4.5" => "Asia/Kabul",
"5" => "Asia/Karachi",
"5.5" => "Asia/Calcutta",
"6" => "Asia/Colombo",
"7" => "Asia/Bangkok",
"8" => "Asia/Singapore",
"9" => "Asia/Tokyo",
"9.5" => "Australia/Darwin",
"10" => "Pacific/Guam",
"11" => "Asia/Magadan",
"12" => "Asia/Kamchatka"
);
return $timezones[$offset];
}
?>
... and use if for conversion :
date_default_timezone_set(offsetToTimezone($hours));
By the way, I suggest you have a look at this answer, which provides you with a more elegant way to achieve the work of offsetToTimezone.
Now that your script if configured on the correct timezone, just ask for a timestamp :
$timestamp = mktime(0, 0, 0);
If at some time, you need to reset to timezone to default, you might need date_default_timezone_get to save it :
$timezone = date_default_timezone_get();
// Change to another timezone based on your offset.
// Get your timestamp.
date_default_timezone_set($timezone);

Answer 3

I had to think through it quite a bit, but I think this was the solution I was looking for. Let me know if you think this algorithm is incorrect.
function getLastMidnightForOffset( $minuteOffset ) {
$today = mktime( 0, 0, 0 );
$tomorrow = $today + 86400;
$yesterday = $today - 86400;
$offset = $minuteOffset * 60;
if ( time() + $offset >= $tomorrow ) {
$localMidnight = $tomorrow - $offset;
} elseif ( time() + $offset >= $today ) {
$localMidnight = $today - $offset;
} else {
$localMidnight = $yesterday - $offset;
}
return $localMidnight;
}