PE 2020: restore default timeline numbering

In PE 2020, I am accustomed to working with a timeline that shows actual time. For instance, I might mark a segment that runs from 1:20 to 1:35. Somehow, unfortunately, PE or I have managed to replace the timeline's time entries with numbers. So now a segment might run from 11094 to 13313. What are these numbers called, and how can I get the timeline back to normal?