Is there a way to access a topic's sort order number?

Good question, though ideally you would have both options.

Also, while we're on the subject of topic attributes that MindManager "knows" about but which aren't directly accessible (like topic sort order and level), I assume that MM must also have an internal date numbering system for topics which it can use to calculate when tasks are due or overdue as well as duration, etc. However, this internal system is very tightly held. Is it possible to access these "secret" date numbers?

Hello Alex. The date information used for tasks is visible in the Task Info task pane. MindManager does not expose dates such as latest start, earliest start etc. found in specialist project management tools. It is possible that these are derived by the internal scheduling algorithm but are not accessible, even in the API. Task dates can be accessed by SmartRules but are not exposed as properties or capable of manipulation in formulas.

Thanks Nick. I wasn't talking so much about things like latest start or earliest start, but simply the task start and due dates themselves. I was hoping that these were based on some sort of underlying numeric date system like you have in Excel, but sadly it seems not.

No they are just date strings. A macro could convert them to and from a numeric property such as Excel's (days from 1/1/1900).

That’s an interesting thought. Would that be difficult to do?

It would be quite a powerful tool - for example, you could compare task due dates to a reference date, or to each other even in different branches. Or, going the other way, you could enter a date once in a map and have it populate the due date for specific tasks. You could also use a task date as the starting point for things like interest calculations over time.