Journal Application in D
I'm not entirely sure what you are asking for, but if this were my program I would do a few things differently.

SQLite

I would place the journal entries in a sqlite database rather than raw files and directories. 

D provides an interface to sqlite. Sqlite provides great abstraction over raw files. Keeping your files consistent, and not corrupting them is difficult. sqlite already does the right thing, and more!

Command Pattern instead of switch

I would provide a nicer abstraction for commands instead of a switch statement. This Python code works as a template. 

By implementing a switch statement, each new command increases the cyclomatic-complexity of the main() function. Thus your testing requirements increase exponentially. By breaking out each command to implement an ICommand interface, your testing requirement increases linearly. The visitor pattern might also provide a nice mechanism to process each command.