I want to create a welcome-box on the website where it displays a message. This message depends on the current time. F.e.: if someone comes to the website at 9am, then there's a message: Goodmorning. If he comes on the website in the evening, it says 'Good evening'.
I don't want to use client-based script. I know it can be done with php (of course).
Is this something that can be easily done with Modx Revo? (without writing php) Are there extra's out there in this direction?
Evolution user, I like the back-end speed and simplicity

can't you use a MIGX - TV, where each item is a time-segment with starttime/endtime and the message.
with the getImageList - snippet, you could filter the message by the current time.
I think MIGX would be a good option.
You could also easily modify the Seasons snippet here to use time segments:
http://bobsguides.com/blog.html/2013/11/21/change-your-modx-wed-site-with-the-seasons/