Calculated value for difference between two dates, with IF qualifier

Hi all,

First time user of the forum and not a Lists expert, so please forgive any errors. See screenshot for example. I have a column in a list that calculates the value between two dates to give the number of days a task was open. However, users only input the second date when they close the task, and tasks are often open for many days/weeks. As a result, the simple formula I have used -

[Task Closed]-[Date Received ] - is giving a minus amount for days open which makes reporting difficult. 

I have a solution in mind but cannot find the suitable formula. Does anyone know of one, or have any other suggestions? Solution idea is:

- Include an 'IF' qualifier so that the column only populates if 'Task Closed' has been completed.

Thanks, Meg