R' Aharon Kotler zt"l commented to a student on the occasion of the birth of the student's son about the phrase "The beris should be be'ito ubizmano", using both "eis" and "zeman" to denote its proper time. Similarly the famous words of Koheles, "Lakol zeman va'eis... everything has its zeman and its eis..." Rav Aharon explained the difference. If the baby is healthy, then the beris is at the pre-decided time, on the eight day.[Eis]. If not, then it will be at the right time for that individual baby.[Zman]. Ideally the beris would be at both.[Be'ito ubizmano].
An eis is a time that comes according to a prescheduled appointment, ready or not. It is a point in a shanah, in cyclic time that runs its celestial heartbeat regardless of human action. A zeman is a landmark in the course of progression. And so, one is "kovei'ah itim baTorah", one sets aside times for Torah.
The above was taken from http://www.aishdas.org/asp/mikeitz64.shtml see the rest of that illuminating article.
Hope this helps!