Luteal is a private, offline-first menstrual cycle tracker that clearly separates recorded facts from calculated estimates. Key features: - Offline by default: Your data stays in a local database on your device with no account required. - Honest tracking: Distinguishes recorded observations from statistical estimate ranges. No medical claims or automated diagnoses. - Complete observation log: Record flow intensity, pain, mood, energy, physical symptoms, and private notes. - Biomarker tracking: Log basal body temperature (Celsius and Fahrenheit) with disturbance tracking and thermal shift calculation. - Cycle history and reports: Interactive calendar, longitudinal variability analysis, and clinician consultation summary export (PDF/HTML). - Privacy and security: PIN and biometric app lock with screen masking. - Duo companion: Private, granular partner sharing with end-to-end encrypted support requests and contextual guidance. - Optional cloud sync: Optional end-to-end encrypted synchronization with self-hosted or community servers. - No ads, analytics, or trackers: 100% free and open-source software (GPLv3).