As far as I can tell, StopWatch does not use the GC at all - it should be fairly simple to make it @nogc.
Commits pushed to master at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/41de4f17b71edfe8101a16b696c23ed0ab89fb4c fix issue 15991 - std.datetime.StopWatch is not @nogc https://github.com/dlang/phobos/commit/88966f953c185c552615b5fda8766cc9211e63c5 Merge pull request #4333 from BBasile/issue-15991 fix issue 15991 - std.datetime.StopWatch is not @nogc