Choose your time format in Immortal Settings ▸ Clock: Auto (follows your Portal's system 24-hour setting), 12h, or 24h.
The choice applies everywhere the time is shown — the home-screen header clock, the screensaver, and the hourly forecast labels (e.g. 13 instead of 1 PM). Auto by default, so existing setups are unchanged; the launcher and screensaver re-read it on resume, so it takes effect as soon as you return home.
Devices on 1.29 and earlier will pick this up on their next self-update check.