Update: Prof. Duncan Temple Lang, one of the core developers of R, actually gave a simpler way of achieving the same thing:

invisible(addTaskCallback(function(...) { 
                 options(prompt = format(Sys.time(), "[%H:%M:%S] >"))
                 TRUE
                 }))

That almost subsumes the functionality of this package, with one minor shortcoming:

The only difference is that when you hit return at the prompt with no expression, the prompt doesn’t change (since there was no task).

I wrote a small plugin for R which can include the current time in the R prompt. It can be found on github.

To use it, run the following commands on R console:

# If needed:
# install.packages('devtools')
library(devtools)
install_github('musically-ut/extPrompt')
library(extPrompt)

I often find myself accidentally starting a R command without realizing how long it will take to execute. Only after the command has been running for some arbitrary time do I realise that I should have at least wrapped it inside a system.time to get an idea of how long it takes to run.

This plugin solves the problem by prepending the approximate start time in form of time-stamps to each command. Now by merely looking at the time-stamps on the R console, one can make a good estimate of the running-times of each command.