I am pretty sure that there is no way via the Admin console to set the time of day for the delivery of subscriptions. Even some of the really primal back-end configuration settings that are only controlled by Khoros probably do not have such flexibility (though it might be worthwhile to open a Support ticket and inquire as to whether there is config setting that can be tweaked).
What I do imagine is that all of the timed delivery of daily digests and the like pivot off of a master clock that is associated with your Community's timezone that you set. That is not to say though that this type of configuration should not be in the product. The more that I think about it, the more I think you should have this ability.
If you like, you can start a new 'Idea' in our Product Suggestions area, or perhaps you may feel inclined to double-down on these two Ideas I found that are at least quasi-related to your request:
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