Imported blender object results in keyframe changes

I need some advice regarding animations. I have searched a fair bit in the forums and answers but have not seen anything that covers my query.
I create a character with animation in blender. The key frames run from 0 - 27. It is imported as a .blend file (had nothing but trouble trying to use .fbx). Everything seems fine but under the animations tab in the import settings it has changed the start frame to -1 and the end frame to 26.
I would have left it but am now getting a message saying that due to negative keyframes animation compression is not possible.
The message only occasionally shows up and I do not understand why when no changes have been made to the scene.
I have tried changing the start and end keyframes to match the original blender files i.e 0 and 27 but it seems to think that frame 27 does not exist and resets the end frame to 26.
I am at a loss as to whether this is normal when importing objects and whether it will affect performance.
Any advice would be useful.

In Blender per default you doesn’t set a key frame 0. (At start of Blender it is set to key frame 1.)
But Unity’s animation’s begin with key frame 0.
This is also setted by default in the importer of Unity to push the animation 1 key frame back so the key frame 1 (Blender) becomes key frame 0 (Unity).

Because of that you should push your animation to begin with key frame 1.