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<?php

/* Notes:  

In PHP dates are stored as numbers.  The number indicates the number of seconds that have passed since January 1 1970 00:00:00 GMT

*/

// 1) Get the current date
$currentDate = time(); // simple method that gets todays date and time

// 2) Get the date of installation
$installationDate = gmmktime(1, 1, 1, 4, 1, 2012); // In this example it is 01:01:01 on 1/4/2012 but I am just working-out how to get this from the date field in EE

// 3) calculate the number of days that have passed
// gets the difference in seconds
$secs = $currentDate - $installationDate;

// get days by dividing
$days = ($secs / (60 * 60 * 24) ); // 60 secs, 60 mins, 24 hours per day

// this will be a decimal, e.g. 10.56 days, so lets round down
$days = floor($days);

// 4) Get the annual savings
// Below is kWh savings
$savings = {embed:annual_kwh_savings};

// 5) Divide the annual savings by 365
$dailySavings = $savings / 365;

// 6) Multiply the result of 5) by the result of 3)
$toDateSavings = $dailySavings * $days;

// get rid of any decimal places
$niceFormat = number_format($toDateSavings, 0); // decimal point and thousands seperator are standard with this method

// 7) Show the final result of 6) 
print $niceFormat . " kWh";

?>