如何从PHP中减去时钟时间的十进制时间

Question

I have the following PHP script:

<?php
  $clockTime = "10:00 AM";
  $decimalTime = "0.75"; // represents time in hours in decimal format

  echo date( 'g:i A', strtotime( strtotime( $clockTime ) . ' -'.$decimalTime.' hours' ) ); // returns "7:00 PM" when I need it to return "9:15 AM"
?>

How can I get the script to properly calculate and return 9:15 AM rather than 7:00 PM?

Answer 1

strtotime returns the time in seconds, so you need to convert the decimal time in seconds:

<?php
date( 'g:i A', strtotime( $clockTime ) - $decimalTime * 60 * 60 ) ); 
?>

However, this will not work when daylight saving (DST) comes into play. Especially if your code will be run in different countries, use timezones and the DateTime-API:

<?php
$date = new \DateTime($clockTime); // This uses the system default timezone where the server is located
$date->sub(new \DateInterval('PT' . ((int) $decimalTime * 60 * 60) . 'S'));
echo $date->fomat('g:i A');
?>