system/View/Filters.php
public static function date_modify($value, string $adjustment): string
	{
		$value = static::date($value, 'Y-m-d H:i:s');

		return strtotime($adjustment, strtotime($value));
	}
public static function date_modify($value, string $adjustment): string
	{
		$value = static::date($value, 'Y-m-d H:i:s');

		return strtotime($adjustment, strtotime($value));
	}
Given a string or DateTime object, will return the date modified
by the given value. Returns the value as a unix timestamp

Example:
my_date|date_modify(+1 day)

@param $value
@param string $adjustment

@return   string
@internal param string $format