By saying ‘还有几天’, you imply there is enough time given to finish the task before the deadline. By saying ‘只有几天’, you show your worries about the coming deadline. It's going to run out of time. 距离他来还有28天。 （If you want to suggest 28 days is a long time to wait） 距离他来只有28天。 （If you are trying to suggest his visit is drawing near but you're not ready for it for some reasons.）

if you say only 28 days,it means time is not sufficient to do sth,and you say still 28 days ,that means it can be used to do you want to do that!!! ALL right????

when you use 还 , it maens you think x days are too long. when you use 只 , it means x days are too short.

"只有x天'’It emphasize the emotion that you only have several days.

距离他来只剩下28天了。