[onedognight]

What you want is WolframAlpha.com. Your examples[0][1] work out of the box.

[0] https://www.wolframalpha.com/input?i=4+weeks+%2B+59*3+hours+...

[1] https://www.wolframalpha.com/input?i=1%2F2+*+36g+*+%28900+m%...