What date and time functions do you know?

• date(): Formats local date and time.
• strtotime(): Converts a date/time string to a Unix timestamp.
• mktime(): Returns the Unix timestamp for a given date.
• time(): Returns the current Unix timestamp.
• getdate(): Returns an associative array of date and time information.
• idate(): Formats local time/date as an integer.
• gmdate(): Formats date/time in Greenwich Mean Time based on Unix timestamp.
• gmstrftime(): Formats Greenwich Mean Time date/time according to locale settings.
• setDate(): Sets the date for a DateTime object.
• setTime(): Sets the time for a DateTime object.
• setTimestamp(): Sets the Unix timestamp for a DateTime object.
• add(): Adds a DateInterval to a DateTime object.
• sub(): Subtracts a DateInterval from a DateTime object.
• diff(): Returns the difference (DateInterval) between two DateTime objects.

I also actively use classes DateTime, DateTimeImmutable, and DateInterval for more object-oriented work with dates and times. For example:

<?php
// Creating a DateTime object
date = new DateTime('now');
echo $date->format('Y-m-d H:i:s');

// Adding an interval
$interval = new DateInterval('P1D'); // Add 1 day
$date->add($interval);
echo $date->format('Y-m-d H:i:s');