Summer is lovely. I don't think you'll have to worry about whether it will date or not - people are into naming their children lots of unusual things these days so there'll definitely be people around with weirder names than Summer! If you're really worried, you could give her a more classic middle name and if she really runs into trouble with Summer later in life, she could go by her middle name.

Summer Elizabeth
Summer Olivia
Summer Charlotte

Good luck!