GoodData is not using date facts anymore and you can find yourself in a situation where you need one.
The basic creation of date fact is really simple. Just place Date attribute after SELECT.
The result looks like you would add a date fact to the report.
You can also apply MAX and MIN functions on this metric and the result will react accordingly to the attribute you will be slicing by.
Focus on the values for 01/31/2014 and 02/01/2014 in the second image and compare it to the values in the image below. By using MIN and MAX functions we can determine minimal and maximal value within a month (or you can use week, quarter...).
Voila! Date facts are back.