You may want to create an appointment that is unplanned if you have agreed to complete a job but not for a specific time or date.
There are a number of ways to set up an appointment. You can do this from the quick action button, from inside a projects and also from the diary itself. Click the link below if you are unfamiliar with creating appointments.
Once you have got to the appointment creation page you will see the header named "schedule". From here you can select unplanned.
Once your appointment has been created you can find it located under the 'more' tab inside the diary view.
You can then drag and drop the appointment into the diary by clicking on the four way arrow next to the appointment title.
Carefully place the appointment over the day/time that you wish to schedule it for.