Building this spiffy HTML5/CSS/JS/PHP app took me a little over a week in total from start to finish. I put in what I feel like was a decent amount of work to produce a quality product in a relatively short amount of time.
Some of the challenges I faced while building the app included: learning PHP from scratch, designing the structure of the JavaScript front end, and as in any web development, trying to center that one element inside that one div with CSS. Overcoming my lack of PHP experience was probably the most enjoyable of the challenges. I love learning new languages and the new paradigms and programming constructs they expose. The next most challenging thing I faced was the desiging the JS backbone of the application. I took this challenge head on by iterating fast on my designs until I reached a solution that I was satisfied with. As in all of my software development endeavors, my primary objective for the app was to make it (A) efficient and functional (B) maintainable and readable (C) aesthetically pleasing from a graphic design standpoint and a software architectural standpoint. Because of these high standards, I went through 3-4 full iterations before producing a final product. Though this may not have been necessary for such a "trivial" application, I wanted to show Expensify how I would work on any major software project I am faced with.
In the end, I am really happy with what I came up with and hope you guys enjoy it as well!
If I were to continue to develop this application, I would probably start with the following things: making the forms more useable, form validation, more feedback/error messages, more responsive design (make it mobile)...just to name a few.
N.B. The HTML5 date picker is only supported in chrome, for this reason the app is most useable. One thing I would do given more time would be to add a jQueryUI date picker to remedy this.