Here i calculate the number of days since a certain date in php and use the values for checking how much something costs per day. 

Start Date

Lets add the date that the item was purchaced. In date format Year-Month-Day

$date_start = date_create("2017-10-29");

Today's Date

Get todays date according to the server date with php

$todays_date = date_create(date('Y-m-d'));

Check the difference between the dates

Using the date_diff function we can check the difference between these dates.

$diff = date_diff($date_start,$todays_date);

Output the details

I usually like to add all the data into a variable and then return it so here we will add all the data to the variable $template_result. This will give us the amount of days since the original date.

$template_result = 'Number of Days Since 29th October 2017 is '.$diff->format("%a");

More Calculations

Using this number we can calculate different things like how many years or months since a date.

Years Since Calculation

Calculate the Years since the date.

$years_calc = $diff->format("%a") / 365;
$years_calc = number_format($years_calc, 2);
$template_result .= "<p>Years $years_calc </p>";

Cost per Day

If the item has an original value we can then calculate the cost per day using the original number.

$cost = 331;
$per_day = $cost / $diff->format("%a");
$per_day = number_format($per_day, 2);
$template_result .= "<p>Original Total Cost: $$cost, Cost per Day $$per_day </p>";

Return the results

Now we can just return the results or just echo them. 

echo $template_result;

You can see a working demo below.