You set a post for 19:00. You fly somewhere three hours away. Your phone changes timezone, and now your carefully chosen evening slot is at four in the afternoon for the audience it was written for.
Why this happens
Most tools store a moment in time and display it in whatever zone your device is currently in. That is correct behaviour for a calendar appointment — the meeting happens at one instant, and you want to see it in local time.
A social post is the opposite. It is not an appointment; it is an intention about your audience's evening. Storing only the instant loses the intention.
The fix
Store the timezone alongside the time, as a property of the post rather than of the viewer. Then 19:00 Europe/Kyiv means 19:00 in Kyiv whether you are in Kyiv, Lisbon or Tokyo.
This also handles daylight saving correctly, which storing a fixed UTC offset does not. An offset of +3 is right for half the year and wrong for the other half; a zone name is right always.
One audience, one zone
If your audience is in one country, set that country's zone once and forget it. If you genuinely have two audiences in different regions, that is an argument for two schedules — not for splitting the difference and posting at a time that suits neither.