What’s a reliable way to sync an offline media tracker across devices?

An append-only operation log is probably the simplest robust solution here. Each device records operations locally and numbers them with its own monotonically increasing counter. During sync, devices exchange operations the other side has not seen, then apply them according to your conflict rules.

This works offline, avoids clock drift entirely, and makes it easier to inspect or undo changes later. You can track which device counters have already been received instead of relying on one global database version, since a single global version can miss concurrent edits made on disconnected devices.

For a personal tracker, network time synchronization will often keep clocks within a second or two, but wall-clock time still shouldn’t be your only source of truth. A device can be offline for a long time or have an incorrectly configured clock, and even a small difference can misorder two close edits.

A safer design is to give every device a unique ID and maintain a counter for its local changes. Synchronize changes using a deterministic ordering such as the counter plus device ID. If two edits conflict, every device applies the same tie-breaker, so they all eventually converge without trusting system time.

Logical clocks or a CRDT are the more formal options if you need automatic merging of concurrent updates. A hybrid logical clock can preserve useful time ordering while handling clock skew, while a CRDT defines merge behavior so replicas converge automatically.

For a small media tracker, though, an operation log with per-device counters is usually enough. If edits are only simple fields such as watched status, rating, or notes, define a clear rule for conflicts—for example, last operation by deterministic device and counter order, or merge sets where possible.