Please do not use DOM manipulation as this is not best, or even good practice.

You can find the angular module here: <yourinstance>.service-now.com/scripts/app.$sp/directive.spDatePicker.js

Here you can see, the {0} is replaced by scope.field.label

In your HTML of sp-date-picker, you have a property called field (usually an angular object, the example its c.fromDate). If you give the c.fromDate object the label property, you can control whats entered in {0}