public function cal_exp_date($input_format)
{
    // Setting default timezone to Asia/Kolkata
    date_default_timezone_set('Asia/Kolkata');

    // Getting current date and time in 'Y-m-d H:i' format
    $C_date = date('Y-m-d H:i');

    // Creating a DateTime object with the current date and time
    $date = new DateTime($C_date);

    // Adding the interval specified by $input_format to the current date and time
    $date->add(new DateInterval($input_format));

    // Formatting the resulting date and time as 'Y-m-d H:i'
    $expire_date = $date->format("Y-m-d H:i");

    // Returning the calculated expiry date
    return $expire_date;
}

• Input Validation: Ensure that the $input_format is a valid format accepted by DateInterval. Otherwise, the function might throw an error or produce unexpected results.

• Error Handling: Consider adding error handling in case of any issues with date/time manipulation.

• Documentation: It's always a good practice to add comments or documentation to your code, explaining what each part does. It helps other developers (and yourself in the future) understand the code better.