It's always a good idea to backup your world save before using a new mod, in case there are any issues. One option is to use Single Player Command (search the forum.) When you use the command " /time day " (without quotations) it will say: "The current time is 06:00 on day XX "
I don't think there is any feature like that or else I would also be interested in that for my Ultra Hardcore Minecraft series on YouTube.
I think he means real Minecraft days and not the current time. If you only want to see the current time in your world at the moment, then you can easily make a clock for that although it isn't as precisely as Single Player Commands.
I hope this post helped you
He wrote on day xx it can show on which minecraft day you are...