If it's a Windows machine, you can run a fairly rudimentary batch file to do this. Create a file, e.g. pingtest.bat, and insert the following lines in it:
@echo off
:START
echo.|date >> log.txt
echo.|time >> log.txt
ping a.b.c.d >> log.txt
goto START
Basically this will capture the date and time, run 4 pings and repeat. It will save all the output to a new file called log.txt
If you want to run more pings than the default of 4, simply add a -n flag (e.g. ping -n 10 a.b.c.d will run 10 pings) to the ping command above. If you run a continuous ping (-t), the batch file will never loop and thus you will never be sure what time it went down.
Hope this helps