Often, you'll go looking for a shell command that you used in the past - you can remember around when you did it but of course that won't help you because by default the bash history won't show you dates/times. Here is an easy way to remedy that. It won't go into the past and give you dates, but from this point forward you'll know when commands were executed.
There are two ways to do this: globally or per user
Globally: edit /etc/bashrc
Per user: edit ~/.bashrc
Add this to the bottom of the file to add date/time:
Now, when you type history, it will show this:
export HISTTIMEFORMAT="%h %d %H:%M:%S "
Now let's say that you want more history to be kept - by default it's set to 500 lines. Add this to the bottom to increase it to 5000 lines:
340 Jan 20 08:26:40 cd /etc/
341 Jan 20 08:30:07 ls -l|grep conf
342 Jan 20 08:35:22 cp resolv.conf resolv.conf.bak
343 Jan 20 08:35:43 vim resolv.conf
Let me know how it works out for you!