Physiological processes oscillate in time. Circadian oscillations, over approximately 24-h, are very important and among the most studied. To evaluate the presence and significance of 24-h oscillations, physiological time distributed data (TDD) are often set to a cosinor model using a wide range of irregularly updated native apps. If users are familiar with MATLAB, R or other programming languages, users can adjust the parameters of the cosinor model setting based on their needs. Nowadays, many software applications are hosted on remote servers running 24/7. Server-based software applications enable quick analysis of big data sets and run on a wide range of terminal devices using standard web browsers. We created a simple web-based cosinor application, Cosinor.Online. The application code is written in PHP. TDD is handled using a MySQL database and can be copied directly from an Excel file to the webform. Analysis results contain information about setting the 24-h oscillation and a unique ID identifier. The identifier allows users to reopen data and results repeatedly over one month or remove their data from the MySQL database. Our web-based application can be used for a quick and simple inspection of 24-h oscillations of various biological and physiological TDD.