Calculate the difference between twice using php, then add it to a t ...

Hi I have a problem where I am trying to get a total time that a job has taken (using database fields job_start and job_end time(7)) then adding this to another model's field,

the code i have is

if (isset($_POST['Jobs'])) { $model->attributes = $_POST['Jobs']; $model->setScenario('closejob'); $model->status = 2; //set status to closed //date time difference - this is the part I need help with $diff = $model->job_start - $model->job_end; //need to get customer model and add time diff to it $customermodel = Customers::model()->findByPk($model->customer_ID); $customermodel->total_time = $customermodel->total_time + $diff; $customermodel->save(); if ($model->save()) $this->redirect(array('view', 'id' => $model->job_ID)); }

I have tried string to time and other date functions but to no avail , the above code throws the following error

CDbCommand failed to execute the SQL statement: SQLSTATE[22007]: [Microsoft][SQL Server Native Client 11.0][SQL Server]Conversion failed when converting date and/or time from character string..

Any ideas on the proper way to do this ? i.e am I going about the adding of time and calculating the differences all wrong ?

I think it has something to do with converting the string to a time format but I am unsure how to do this

Maybe you have to do

$diff = $model->job_end - $model->job_start;

Instead of

$diff = $model->job_start - $model->job_end;