[hakarrr]

Timer(start_time=0) silently selects the automatic monotonic timer instead of measuring from the Unix epoch. A monotonic reading of zero also stops automatic and resumable timers from accumulating elapsed time.

Use None for an inactive resumable timer and check explicitly for None, so zero remains a valid start time. This also keeps a stopped timer from counting time between resumptions.

• Three deterministic test methods: epoch start (zero, negative and positive controls), zero monotonic start, and paused/resumed lifecycle. Two fail on the unchanged base; the lifecycle control passes.
• Full suite: 72 passed on each of CPython 3.9, 3.10, 3.11 and 3.12 on Linux.
• Combined line/branch coverage: 91%; changed Timer branches covered.
• Changed-file flake8 (including docstrings), diff check, sdist and wheel: pass.

Python 2, earlier Python versions, Windows and macOS were not run. No test or CI configuration changes. Found by reading the timer implementation; no separate reported production incident.