Yes, if you have two articles by the same author but they are published in different years, then this is sufficient in your in-text citation to distinguish between them.
If you had two different articles published by the same author in the same year, you would need to add something additional to the in-text citation to distinguish between them. In the Harvard style, they do this by adding a letter after the year.
For example, (Allan 1998a)......(Allan 1998b).
Please see the Harvard referencing information on our website http://libguides.library.usyd.edu.au/content.php?pid=160012&sid=1510179