Once I had a similar problem and found this explanation on the webpage of the journal corresponding to the paper I wanted to cite:
How do I cite papers that appear in Advance Access?
Papers published in Advance Access are citeable using the DOI and publication date. An example of an Advance Access citation is given below:
Gilad, Y. and Lancet, D. Population Differences in the Human Functional Olfactory Repertoire. Mol. Biol. Evol. Advance Access published March 5, 2003, doi:10.1093/molbev/msg013
The same paper in its final form would be cited:
Gilad, Y. and Lancet, D. Population Differences in the Human Functional Olfactory Repertoire. Mol Biol Evol 2003; 20: 307-314. First published March 5, 2003, doi:10.1093/molbev/msg013