add a check for empty Calendar before trying to loop through all tasks.
Merge branch 'master' of https://github.com/mymichellle/TT
tutorial_studyTime.tt update and test
Fix every Task t on (t ...) so t is valid in the check
Add/subtrackt timeframes
Merge branch 'master' of https://github.com/mymichellle/TT
Update tutorial_daysLeft.tt to use getCurrentTime()