[Scandal]

A "fast" way to do it is the following (no plugins or addons etc):
Open template "postbit_legacy" and place the following code:

<dt>Registration Days</dt> <dd><vb:if condition="$post[sc_joindays] =  floor(abs((TIMENOW - $post[joindateline])/60/60/24))"></vb:if>{vb:raw post.sc_joindays}</dd>

During the save of the template you will get an error, but ignore it and click Continue.

I have not test it much, just I wrote it fast for now.