8:00. In other words, your project deadline remains. However, instead of pre-assigning errands, each agrees to start with one errand and then call the other to coordinate what should be done next. This way, if you're running behind, your spouse can take on a time-consuming errand while you do a shorter one. The schedules can be more perfectly balanced due to adjustments made in real-time. This type of continuous planning prevents either of you arriving at the restaurant thirty minutes before the other.
The types of changes discussed here are commonly used in many Agile processes. These changes allow a team to produce more software faster than if a more prescriptive process were used. Keep in mind, there is more to Agile planning than the simple changes outlined in this article. But, if you make these simple changes to your plans, you're off to a good start.