In your tender, go to the 'Planning' tab.

Select the bottom right of the current planning for 'Add date/phase'.

In the pop-up window, enter the information about the event you'd like to add to your planning. Adding a date you only need to specify a start date. This can be used for, for example, for single deadlines or milestones.

Adding a phase you have to specify both a start and an end date in order to inidicate the period within your planning.

A new date / phase can only take place in the future.

The 'Add date/phase' button not visible? 

Check whether your tender is not 'archived',
- You have been assigned sufficient user-rights by the Leadbuyer for amending the planning?
- Are you already in the 'Quick Edit' mode? First save you latest amendments and then try again.