Reddit's Globalization over Twenty Years: Inferring Community Time Zone from Activity Timestamps

Online communities are global, but measuring their actual geographic spread has been challenging without user disclosures. A new paper from Franco Della Negra, Mattia Samory, and Matteo Cinelli (arXiv:2605.04371) proposes a scalable method that infers a community's time zone solely from its temporal activity patterns—requiring nothing beyond hourly post counts. Grounded in the circadian rhythm of posting behavior, the researchers compared time-domain and frequency-domain techniques against a simple heuristic: activity reaches its minimum around 4 a.m. local time. On Reddit, the best-performing method accurately pinned down a community's time zone to within 30 minutes, and the heuristic alone recovered the correct zone within a one-hour margin on average. Remarkably, fewer than 1,000 comments were enough to achieve peak accuracy, making the approach viable even for small or niche subreddits.

The authors validated the method across communities as varied as sports and finance, demonstrating generalizability beyond cultural boundaries. Applying it at scale, they characterized Reddit's geographic evolution from its founding in 2005 through 2025, revealing how the platform globalized over two decades. The method is portable to other platforms and requires no user disclosure, offering a privacy-preserving baseline for any study needing to account for the geographic structure of online behavior. This work provides a practical tool for social scientists, platform analysts, and researchers studying the spread of digital communities across time zones.