Sorry for my delayed answer. But I don't see it either :/

"The prompt will default to a timer string at
+  point (if any),"

I can't see it well related to the C-u prefix. For example why the "(if any)"? It should be unnecessary, since it is the previous explained general case.

Best regards

Ypo <ypuntot@gmail.com> writes:

Oh! Thanks

I didn't understand. Maybe this would be more understandable?:

  When called with a ‘C-u’ prefix, prompt the user for a starting
     offset or, if there is a timer string at point, this is taken as the
     default, ...

Or:

  When called with a ‘C-u’ prefix, there are 2 options: prompt the user for a starting
     offset.  Or, if there is a timer string at point, this is taken as the
     default, ...

Your versions still feel a bit confusing.
What about the attached patch?