DateTime

Get current date time:

// DateTime($time='now', DateTimeZone $timezone=null)

// Usage
$now = new DateTime();

sub

Subtracts an amount of days, months, years, hours, minutes and seconds from a DateTime object

$date = new DateTime('2000-01-20');
$date->sub(new DateInterval('P10D'));
echo $date->format('Y-m-d');

add

Adds an amount of days, months, years, hours, minutes and seconds to a DateTime object

$date = new DateTime('2000-01-01');
$date->add(new DateInterval('P10D'));
echo $date->format('Y-m-d');

DateInterval

The format starts with the letter P, for period. Each duration period is represented by an integer value followed by a period designator. If the duration contains time elements, that portion of the specification is preceded by the letter T.

Examples:

Period Designator	Description
Y	years
M	months
D	days
W	weeks. These get converted into days, so can not be combined with D.
H	hours
M	minutes
S	seconds

Here are some simple examples. Two days is P2D. Two seconds is PT2S. Six years and five minutes is P6YT5M.

Loop through DateInterval

$installments = 18;
        
//current date
$now = new DateTime();
//30 days interval between each installment
$interval = new DateInterval('P30D');

//period
$period = new DatePeriod($now, $interval, $installments, DatePeriod::EXCLUDE_START_DATE);

foreach ($period as $date)
    echo $date->format("Y-m-d"), '<br>';

Create a Date From a String

Here, we will be using PHP's strtotime()

The PHP strtotime() function is used to convert a human readable string to a Unix time.

strtotime(time,now)

The following example will give you a better idea of this function

<?php
    $t = strtotime("11:45am October 16 2019");
    echo "Created date is ".date("Y-m-d h:i:sa",$t);
?>

Using this function, you can parse about any English textual datetime description into a Unix timestamp. Here are some examples

<?php
    $t=strtotime("tomorrow");
    echo date("Y-m-d h:i:sa", $t) . "<br>";

    $t=strtotime("next Thursday");
    echo date("Y-m-d h:i:sa", $t) . "<br>";

    $t=strtotime("+2 Months");
    echo date("Y-m-d h:i:sa", $t) . "<br>"; 
?>

Create a Date with PHP mktime()

The mktime() function returns the Unix timestamp for a date. The Unix timestamp contains the number of seconds between the Unix Epoch (January 1 1970 00:00:00 GMT) and the time specified.

<?php
    mktime(hour,minute,second,month,day,year)
?>

Here is an example to create a date and time with some parameters in this function

<?php
    $t = mktime(11, 14, 54, 8, 11, 2019);
    echo "Created date is " . date("Y-m-d h:i:sa", $t);
?>