Library for text substitutions with calculations?

[Jan Erik Moström]

I want to do some text substitutions but a bit more advanced than what 
string.Template class can do. I addition to plain text substitution I 
would like to be able to do some calculations:

$value+1 - If value is 16 this would insert 17 in the text. I would also 
like to subtract.

$value+1w - In this case value would be a date, this would add a week to 
that date, similar for year, month, hours. Also subtract.

The exact syntax isn't important.

In other words I would like to define

example1 = 12
example2 = 2020-12-15

and then some text:

Hello world $example1+10 and a date $example+1w

would result in

Hello world 22 and a date 2020-12-22

Is there some library that can do something like this?

I probably searching using the wrong keywords because I can't find 
anything.

= jem