Setting the story in a near-future world makes sense. Protagonist could be someone tech-savvy, maybe a programmer or hacker. The portable aspect could mean the device is a wearable or a gadget they carry around. Timekeeping could involve time travel, time manipulation, or even a metronome effect on reality.
Potential pitfalls: Avoid clichés of time travel stories. Add unique twists, like time being a finite resource or changes to the timeline having a ripple effect not on time itself but on reality's physical state. i--- Portable Timekeeper 1.42 Download
Characters: Protagonist could be someone with a personal loss, like a deceased relative. The app allows them to revisit the past. They meet someone who can guide them or warn them of dangers. Setting the story in a near-future world makes sense