Redmine#3251: document GMT hard time classes and sys.cdate/sys.date

(cherry picked from commit 171b4d0f66fc4bc5c94dbefc53593165c50a1a48)
classes.markdown: Fix there -> these typo

(cherry picked from commit cc157bae73a3cee3e62a642580be2ac6cb24ff2c)
no license needed for 25free

(cherry picked from commit 204b69c20681099737ea7d4f5955d109f46d18de)