Something's come up. You need q dollars by a certain date. A gift giving occasion, vacation or a repair bill. It is simple to look into the future if you manage your bills using Google Calendar. Do the following...
Start with your current bank balance
Add income for the time period in question
Go through your calendar subtracting bills until D day
You may want to work it out until the next pay day just to be safe.
I usually just use a calculator for this. But sometimes I will write it out day by day.