In PHP there are some Pre-defined or built in Date/Time functions.

checkdate
  -Validate a Gregorian date

date_add
  - Alias of DateTime::add

date_create_from_format
  - Alias of DateTime::createFromFormat

date_create
  - Returns new DateTime object

date_date_set
  -Alias of DateTime::setDate

date_default_timezone_get
  -Gets the default timezone used by all date/time functions
   in a script

date_default_timezone_set
  -Sets the default timezone used by all date/time function
   in a script

date_diff
  -Alias of DateTime::diff

date_format
  -Alias of DateTime::format

date_get_last_errors
  -Alias of DateTime::getLastErrors

date_interval_create_from_date_string
  -Alias of DateInterval::createFromDateString

date_interval_format
  -Alias of DateInterval::format

date_isodate_set
  -Alias of DateTime::setISODate

date_modify
  -Alias of DateTime::modify

date_offset_get
  -Alias of DateTime::getOffset

date_parse_from_format
  -Get info about given date

date_parse
  -Returns associative array with detailed info about 
   given date

date_sub
  -Alias of DateTime::sub

date_sun_info
  -Returns an array with information about sunset/sunrise,
   twilight begin/end

date_sunrise
  -Returns time of sunrise for a given day and location

date_sunset
  -Returns time of sunset for a given day and location

date_time_set
  -Alias of DateTime::setTime

date_timestamp_get
  -Alias of DateTime::getTimestamp

date_timestamp_set
  -Alias of DateTime::setTimestamp

date_timezone_get
  -Alias of DateTime::getTimezone

date_timezone_set
  -Alias of DateTime::setTimezone

date
  -Format a local time/date

getdate
  -Get date/time information

gettimeofday
  -Get current time

gmdate
  -Format a GMT/UTC date/time

gmmktime
  -Get Unix timestamp for a GMT date

gmstrftime
  -Format a GMT/UTC time/date according to locale settings

idate
  -Format a local time/date as integer

localtime
  -Get the local time

microtime
  -Return current Unix timestamp with microseconds

mktime
  -Get Unix timestamp for a date

strftime
  -Format a local time/date according to locale settings

strptime
  -Parse a time/date generated with strftime

strtotime
  -Parse about any English textual datetime description into a 
   Unix timestamp

time
  -Return current Unix timestamp

timezone_abbreviations_list
  -Alias of DateTimeZone::listAbbreviations

timezone_identifiers_list
  -Alias of DateTimeZone::listIdentifiers

timezone_location_get
  -Alias of DateTimeZone::getLocation

timezone_name_from_abbr
  -Returns the timezone name from abbrevation

timezone_name_get
  -Alias of DateTimeZone::getName

timezone_offset_get
  -Alias of DateTimeZone::getOffset

timezone_open
  -Returns new DateTimeZone object

timezone_transitions_get
  -Alias of DateTimeZone::getTransitions