No, but as a workaround you can create a "No time" task to fill the time frames you don't want to have any plan for. Edit it and tick off the "Enable auto reminder" option so that you don't get any alarms for it.
Note that by default you'll be reminded to continue planning after the end of the plan is reached. This feature may be disabled in the Settings ("Enable planning reminder"). Or the frequency of the reminders may be adjusted ("Nagger frequency"). The default is every 15 minutes.
What if I don't want to plan the entire day, can I have holes in my plan? Periods without plan?
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