They are just simple built-in helper commands that can optionally be bound to external UI buttons. I don't believe we use them anywhere by default.
If trying to unit test with them, your options would be to:
1) Make your own command that sets the edit box's Value property based on a supplied DateTime command parameter. This is the Today one's logic:
this.Value = DateTimeHelper.Range(DateTime.Today, this.Minimum, this.Maximum);
2) Or alternatively change your unit test logic so it checks to see if the value after executing the command is within a second of the current DateTime (for SetValueToNowCommand), or within a day of the current date (for SetValueToTodayCommand). That way edge cases like being executed right at midnight, etc. are handled.