Full disclosure: This was a client of mine a long time ago.
Don't have much advice as far as what to use and certainly don't want to discourage you. Python might be good. Maybe Java or something if you want it to be mobile. Honestly, you could probably make the whole thing with HTML/css/js/sql.
I use OfficeTime which is for Mac/Windows/iOS has a lot of great functions in addition to being able to just stop/start a clock when you're working. You can set multiple projects, categorize the types of work you're doing if you want to set a different rate. It'll also make invoices and has lots of other functions I haven't even tried yet.
Also, it notices when you're away from your computer for long stretches of time, so if you forgot to turn off the timer you can subtract the time or if you were rendering or something else you can keep the time.