Libreoffice: create multiple rows from 1, depending on row content

Question

I have a style sheet in Libreoffice, like this:

       DateA | DateB | DateC | DateD
---------------------------------------
John |     1 |     1 |     0 |     1
Mary |     0 |     1 |     0 |     1

What I want is to create a new style sheet, like this

     | Date
---------------------------------------
John | DateA
John | DateB
John | DateD
Mary | DateB
Mary | DateD

In other words: create a row per person and per date. The result would be multiple rows per person, whereas in the original file each person has one row with multiple dates.

There are similar questions about Excel (like this one), but unfortunately I only have Libreoffice. Any help is appreciated.