Go over umask the system call, but exemplify it with umask the shell command.
Do both files and directories.

Talk about system calls that are described in the lecture notes.

Go over time(), localtime(), asctime() and mktime() -- 
   Open img/struct_tm.jpg and explain.
   Open img/img/time_calls.jpg and go over:

Go over or write src/t1.c -- time(0), then localtime() to convert & print.
   Add 3600 seconds, and then do it again to a second pointer.
   Now show that both pointers go to the same place.

And src/t2.c -- Show how struct_tm's are convenient because of things like
   leap years.  Add a year 10 times to a struct_tm.