Once you have created a new Solicitation and you want to copy the planning from a previous Solicitation, you can easily use templates.

Go to the original Solicitation you want to copy the planning from (you have to be a participant in the solicitation). Under the third tab 'Planning' you'll find a 'Save as Template' link at the top right part of your screen.

Name and save the planning template.

Now navigate to the solicitation, where you want to use the existing planning. Go to the third tab again: 'Planning' and now choose 'Import template' to load the existing planning into your new solicitation.

Using this option, events are added to the planning and all dates and times are re-used relative to the start date of this new solicitation.

I cannot select the template I've just created? 
If you are not able to re-use the planning, most probably both solicitations have a different procedure with possible additional phase(s).
Look in the original solicitation and see what procedure was used under: Tender Properties > Procedure, and make sure your new solicitation has the same procedure assigned to itThis is a prerequisite for importing a Planning template.

Delete a planning template

Open a solicitation in preparation, go to Planning. Click on Import template and select the template you want to delete. Click on the trash bin icon to delete the template.