clockoverlay: fix bogus time display caused by previous commit

Fixes regression introduced by "clean-up" done as part of commit 98ebcb4b.

dummy must live as long as use the return value of localtime_r() since that's just a pointer to it, and by putting it inside the block we made dummy go out of scope right after localtime_r() returned, which messed up the time values since when we poked at the struct the contents might already have been overwritten.

Fixes #722 (closed)