To convert a string to a number you would want to use the parseInt() function, You should be able to use this in an expression so it would be something like {{parseInt($json["myString"])}}
If you wanted the number to be split with a comma it it just a case of adding .toLocaleString() so you would have {{parseInt($json["myString"]).toLocaleString()}} but as this contains a comma it would no longer be considered an int and would be a string.
The example below shows both of these options in a function node so you can get a feel for what they do.
With the second part of your question the 2 dates look to be the same so I am not sure what it is you want to do with them but for date conversions there is a handy Date & Time node which is well documented and can deal with that sort of thing.