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

Go to the original Tender you want to copy the schedule from (you have to be a participant in this tender). Under the third tab 'Schedule' you'll find a 'Save as Template' link at the top right part of your screen.

Name and save the template schedule.

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

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

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

Delete a schedule template

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