I built a completely offline cycle tracker for my partner because mobile apps felt too risky for her personal data. It has since grown into a real product.

Hey r/SideProject,
I’ve been working on a build that started from a real problem my partner and I were facing. We wanted a simple and private way to understand her cycle so we could plan better. Things like travel, busy weeks, downtime, and just being more in sync with each other.

We tested the big mobile apps like Flo and Clue, but every one of them required cloud sync, accounts, and heavy data collection. The more we reviewed their privacy policies, the more it felt like we would be giving away control over very personal information. That was not an option for us.

We needed something local, quiet, and trustworthy. A tool, not a service.

So I built one.

CycleCore is the result. It is a desktop-first cycle tracking engine focused on privacy and clarity.

There are two versions:

CycleTracker Pro
The version I created for myself. It manages up to 10 profiles and gives a clean forecast of cycles, fertile windows, and ovulation. It has been incredibly helpful for planning and being more intentional in my relationship.

Cycle Harmony
A version designed for personal, privacy-first cycle tracking. All data stays on the user’s device with no external sync of any kind.

Under the hood:
• Built with Python and Tkinter to keep it lightweight and cross-platform.
• Uses local, readable files that the user fully controls.
• No accounts, no servers, and no background syncing. It is fully offline.

I just launched it on Gumroad. The landing page shares the full backstory and a more detailed breakdown.

To celebrate the launch, the first 100 users can get it for 19.99 USD before it increases to 29.99 USD. It is a one-time payment for lifetime access to version 1.x.

This project came from a real need, and I am excited to finally share it with the community.

I would love your thoughts or feedback.
Landing Page: https://365globalsolutions.com/