The Impulse engines that Starfleet uses for non warp travel are relativistic sub-light engines. Full impulse is about 0.25c. They make a point of not using them for long periods of time to avoid time dialation issues. When needed they synchronize the ships time with a nearby starbase to make sure they don’t drift too much.
I think the 0.25c claim is mostly Technical Manual content and not on-screen canon. The manual also claims that the stardate calendar is designed to cope with time dilation and relativity of simultaneity issues.
The Impulse engines that Starfleet uses for non warp travel are relativistic sub-light engines. Full impulse is about 0.25c. They make a point of not using them for long periods of time to avoid time dialation issues. When needed they synchronize the ships time with a nearby starbase to make sure they don’t drift too much.
I think the 0.25c claim is mostly Technical Manual content and not on-screen canon. The manual also claims that the stardate calendar is designed to cope with time dilation and relativity of simultaneity issues.