i agree with previous commenters that your wish is not a self-going task in LO calc,
but there are workarounds …
two which come to my mind just now:
1.) use a ‘link to external data’ and ‘update every xx seconds’ to trigger frequent updating of something in your workbook, and that changing to trigger updating of the timer or counter you want, or
2.) use macro code, e.g.
sub clock1
rem ----------------------------------------------------------------------
rem define variables
dim document as object
dim dispatcher as object
rem ----------------------------------------------------------------------
rem get access to the document
document = ThisComponent.CurrentController.Frame
dispatcher = createUnoService("com.sun.star.frame.DispatchHelper")
for i = 0 to 10000
wait 1000
rem ----------------------------------------------------------------------
dispatcher.executeDispatch(document, ".uno:Calculate", "", 0, Array())
next i
end sub 'clock1
will upate @Opaques wall clock every second for the next 10.000 seconds, it’s not even blocking other work in the sheet,
(be careful, i didn’t check how much of the sheet will be recalculated, just used what macro recorder got for a recorded ‘F9’, could be problematic to recalculate a large sheet too often)
[edit] !!! be careful, nice problems trying to close the program / sheet while macro is running, and even nicer problems trying to restart, save all work beforehand !!! (at least with ver. 7.1.0.0.a0+) [/edit]
there will be plenty of other aproaches, use the search engine of your choice and keywords like libreoffice, calc, time, timer, clock, dynamic, update or similar,
hope that helps … 
