Each to-do item can have 3 different dates around it.
A due date is the day which the item is due. You can change this date by selecting the to-do item options menu and selecting "change due date".
The to-do period is the week which you are going to work on it. Within your To-do, you can see these periods are "Today", "This week", "Next week" and "Later".
Status refers to the progression towards the completion of the item — it can be Planned, In progress, Waiting, Completed or any of your chosen Practice workflow statuses.
Finally, last activity shows the last time the item was updated.
I am assigned a note with a To-do date on June 20th (for all intents and purposes, 20 days from now), so it shows up on my to-do for Later.
The To-do date is June 20th, but I might decide I need to start working on it sooner than that. So I drag the item to This week and am asked to choose a day this week for the new To-do date. I choose Wednesday—June 3rd.
I then add a reply to the note to get help from a colleague. This will update the last activity part of the note, but will have no impact on all other timeframes.
When I tick it off, the due date will remain, the item will appear in Completed, it will show up as Complete (with a timestamp) and the last activity will be the day I marked it as done.