It's not possible to configure time-of-day access on a per-user level. In earlier versions (2.4 I believe) this feature was planned and was even written into the documentation, but it never actually made it in as the decision was made to concentrate on other things.
In 2.6 the number of groups was increased to 500 so there should be enough to put your users into individual groups and define time-of-day on the group basis. Hope that helps.