What is the purpose of a plus symbol before a variable?

The + operator returns the numeric representation of the object. So in your particular case, it would appear to be predicating the if on whether or not d is a non-zero number.

Reference here. And, as pointed out in comments, here.

It is a unary "+" operator which yields a numeric expression. It would be the same as d*1, I believe.

As explained in other answers it converts the variable to a number. Specially useful when d can be either a number or a string function that evaluates to a number.

Example (using the addMonths function in the question):

addMonths(34,1,true);
addMonths("34",1,true);

then the +d will evaluate to a number in all cases. Thus avoiding the need to check for the type and take different code paths depending on whether d is a number, a function or a string that can be converted to a number.