Timestamp sensors do not accept dayjs as state

## 🐜 Bug description

Want to create a sensor to track timestamps of an event, but it seems like the functionality to take in `dayjs()` objects for sensors was never implemented.

Throws some typing errors, but on the internals it looks like implementation was started.

## 🎙️ Additional details

Probably a fixme item?

No files or tests are named. Start by tracing the sensor state types and the existing timestamp handling, then reproduce the reported TypeScript errors with a dayjs() value. Done means dayjs objects are accepted as sensor state without the reported typing errors.

Written by the indexing model from the issue text.