Timewarrior

Runs on multiple platforms and can be easily integrated into scripts, as emphasized in the README's key features for workflow flexibility.

Maintained as a very active open-source project with regular updates, evidenced by GitHub badges showing recent releases and commit activity.

Offers sophisticated calendar-based backfill using a flexible syntax, allowing retrospective time entry without real-time tracking, per the key features.

Designed for simplicity and efficiency in the terminal, enabling quick start/stop timers and reports without leaving the command line, aligning with its philosophy.