Just to add another view, which is much easier, for your next request in line of this one:
Use a scm (like git) like you always do with your projects (right?). Hand over an export of the full repository, including the history, on delivery. This will be
- cheap in terms of time (you already do it)
- cheap in terms of disk space
- easy to make an overview of (number of commits, dates between commits, graphs etc)
- easy to look at steps in "the wrong direction"
- easy for you to cover up steps in the wrong direction (merging and rebasing in git terms) but still keep the time log correct
The rest that's on my mind has already been said.