By default, cards will be 'snapped' to the scale in the Gantt chart as this makes it easier to plan cards in the planning phase and many users schedule cards by day. They get rounded up or down according to the time scale.
Snapping to scale is implemented based on the following logic:
When updating the start date only, the start date will move to the nearest available start of the day based on the set work hours.
When updating the end date only, the end date will move to the nearest available end of the day based on the set work hours.
When updating both start and end date, the card will be moved to the first available start date.
You can decide to plan on a detailed scale and disable this setting, for this open the settings menu and disable 'Snap cards to scale'.
If you like to keep save these settings, refresh the Gantt chart and save changes after adjusting it.
More information on Gantt charts
If you want to learn other Gantt-related features, check out the Gantt section in our help center or ask us using the chat button.