I'm afraid it isn't that simple. When I simply overlaid the default value and then tabbed out of the field, the default value immediately replaced the value I just entered. However, I have discovered a trick. If I don't tab out of the field and then just click "Save and Close", then the value I typed wasn't overlaid.
But suppose I have a bunch of donations. The default value is the previous Sunday, but I want the donated date to be two Sundays ago for numerous payments. I would prefer my value to be locked on the screen in place of the default value. Each time I click Save and "Add New", the default value appears instead of my override value.