php datetime functions

//	Date and time in php

// Create source date/time variables
$sDateTime = '26 Aug 2018  06:30:01';
$sLocalTZ = 'America/New_York';
$iYear = 2018;
$iMonth = 8;
$iDay = 26;
$iHour = 6;
$iMin = 30;
$iSec = 1;

/////////////////////////////////////////////////////////////////////////////////////
// Time zone (always know or establish the relevent timezone)
if (date_default_timezone_set($sLocalTZ))
	echo '
Timezone is '.date_default_timezone_get().'
';
else
	echo '
ERROR - unable to change the default timezone to '.$sLocalTZ.'
';

if (date_default_timezone_set('UTC') == false) die('ERROR - unable to set default timzeone to UTC');


/////////////////////////////////////////////////////////////////////////////////////
// Creating datetime 

// Create unix timestamp (seconds since January 1 1970 00:00:00 UTC) relative to the current time zone
$iUnixTime = mktime($iHour,$iMin,$iSec,$iMonth,$iDay,$iYear);

// Create datetime object from datetime string in the UTC time zone
if (date_create($sDateTime) == false) die('ERROR - date_create()');
$oDateTime = date_create($sDateTime);


/////////////////////////////////////////////////////////////////////////////////////
// Datetime conversions

// Convert datetime string to unix timestamp (seconds since January 1 1970 00:00:00 UTC)
// Conversion is relative to the current time zone.
if (strtotime($sDateTime) == false) die('ERROR - conversion of date failed '.$sDateTime);
$iUnixTime = strtotime($sDateTime);

// Convert unix timestamp to datetime string
$iUnixTime = mktime($iHour,$iMin,$iSec,$iMonth,$iDay,$iYear);
$sUnixTime = date('j M Y  h:i:s A', $iUnixTime);
//echo '
$sUnixTime = '.$sUnixTime.'
';	// output: "$sUnixTime = 26 Aug 2018 06:30:01 AM"
unset($sUnixTime);

// Convert datetime string to another time zone  (UTC to 'America/New_York')
// (Note that the datetime string is derived from the unix timestamp $iUnixTime using the date() function).
$dtLocal = date_tz_convert(date('d/m/Y H:i:s',$iUnixTime),'UTC','d/m/Y H:i:s',$sLocalTZ, 'jS M Y   h:i a');
// echo '
$dtLocal = '.$dtLocal.'
'; outputs "26th Aug 2018 02:30 am"


/////////////////////////////////////////////////////////////////////////////////////
// Displaying datetime 

// Use date() to display a Unix timestamp
$iUnixTime = mktime($iHour,$iMin,$iSec,$iMonth,$iDay,$iYear);
echo '
$iUnixTime = '.date('j M Y  h:i:s A', $iUnixTime).'
';

// Use date_format() to display a datetime object
$oDateTime = date_create($sDateTime);
echo '
$oDateTime = '.date_format($oDateTime,'j M Y  h:i:s A').'
';

/////////////////////////////////////////////////////////////////////////////////////
// Datetime manipulations

// Add one day to datetime variable $startDT
if (date_add($oDateTime, date_interval_create_from_date_string('3 days')) == false) die('ERROR - date_add()');
//echo '
$oDateTime = '.date_format($oDateTime,'j M Y  h:i:s A').'
';	// outout "$oDateTime = 29 Aug 2018 06:30:01 AM"


/////////////////////////////////////////////////////////////////////////////////////
// Custom datetime functions

function date_tz_convert($dt, $tz1, $df1, $tz2, $df2) {
	/**
	 * Convert a date(time) string to another format or timezone
	 *
	 * DateTime::createFromFormat requires PHP >= 5.3
	 *
	 * @param string $dt
	 * @param string $tz1
	 * @param string $df1
	 * @param string $tz2
	 * @param string $df2
	 * @return string
	 
	 https://www.pontikis.net/tip/?id=29
	*/
  $res = '';
  if(!in_array($tz1, timezone_identifiers_list())) { // check source timezone
    trigger_error(__FUNCTION__ . ': Invalid source timezone ' . $tz1, E_USER_ERROR);
  } elseif(!in_array($tz2, timezone_identifiers_list())) { // check destination timezone
    trigger_error(__FUNCTION__ . ': Invalid destination timezone ' . $tz2, E_USER_ERROR);
  } else {
    // create DateTime object
    $d = DateTime::createFromFormat($df1, $dt, new DateTimeZone($tz1));
    // check source datetime
    if($d && DateTime::getLastErrors()["warning_count"] == 0 && DateTime::getLastErrors()["error_count"] == 0) {
      // convert timezone
      $d->setTimeZone(new DateTimeZone($tz2));
      // convert dateformat
      $res = $d->format($df2);
    } else {
      trigger_error(__FUNCTION__ . ': Invalid source datetime ' . $dt . ', ' . $df1, E_USER_ERROR);
    }
  }
  return $res;
}
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