I hope to have it solved....:
Once again my problem: A calendar field only used for dates, not for time. Adding an article with date and after that searching for "future" only shows dates beginning one day after the search day.
To solve that you need a field and a helper field. Important: You have to use the normal field for searching and the helper field for the input form.
If the search is set to "future", you have to set the input (helper) field to a late time, let's say 23:00:00. The search (normal) field must have an earlier time, 00:00:00.
Both fields must have the same storage settings. Using the "normal" field for searching is important, because it uses the correct alias to process the search. The input helper field only uses storage information for saving, the field alias doesn't matter.