[TechDebt] Clean up how we handle dates (the most-recent-sampling date)

[fbielejec]

Problem

Current handling of the dates is a mess, dates are constantly parsed and reparsed beetwen different representations between the client /DB and the worker.

Possible solution

I quite like how we handle CreatedOn - internal representation is always an epoch time, and parsed to a localized date only on the edge (in the browser client).