This script provides a window displaying the current position (in minutes and seconds) like the builtin "Big Clock". But in the rightclick menu it can be chosen from three different times:

Time since project start (the same as Reaper's big clock)
Time since previous marker
Time since previous region

Like that you can easily display, for example, the time since the start of the current song (= region). Additionally you can choose from two different time units:

Hours, Minutes, Seconds and Milliseconds (e.g. 1:23:45.678)
Measures and Beats (e.g. 42.2.00)

Screenshot

Upper window: Time since project start Middle window: Time since previous marker Lower window: Time since previous region