Server side or Javascript calculations?
In addition to the facts stated by thedk, you should also keep in mind that calculations you do on client side are more likely to fail because the client may not fulfill certain preconditions. Think of disabled JavaScript or an unreliable internet connection. You generally have no control over your data as soon as it has left the server.
So, it would be highly advisable to move only unimportant calculations to the client side. Something like datetime formation might be okay, but don't try to parse your whole website with JavaScript. Your website should work (and look acceptable) even if JavaScript is disabled on the client.
In general is it better for performance to do lots of data calculations on the server side or on the javascript side? 
I have a bunch of data that i'm displaying on a page - and I'm wondering if I should format/ parse/ make calculations on that data on the server side (in python) and return a template or if I should return the data as is and do all my calculating/ formatting on the javascript side?
Are there any general rules of thumb when making these decisions?
Examples of things i'm calculating - converting timestamps to dates.