setting a variable from a function called from PS1

[E R]

I've been trying to get a function called from PS1 to set a variable, e.g.:

num=1

function xyz {
 ((num++))
 date; echo "num: $num"
}

PS1="\$(xyz): "

In this example, the global value of num doesn't change if it is
called from PS1. However num does get incremented if xyz is called
directly.

Is there a way I can have a function called from PS1 change the value
of a global variable?