If you want a simple number:
Add Global variables, let's call it Timer
When player starts the game you can add dt to that variable.
When player died, stop adding and save that number into your leaderboard.
If you want a time format: mm:ss
Do the same above.
But you need another string variable to convert the number to time format, let's call it TimeFormat
TimeFormat = floor(Timer/60 % 60) & ":" & floor(Timer % 60)