All birth dates on this page were either the end of 1866 or the beginning of 1867.
With Indexing we type what we see. I think of it this way - we are creating an Index for the researchers. When the researcher finds an entry on an Index it's up to them to have a look at the image and interpret the information. It's not up to us to interpret the information.
So yes, we should index what is recorded.
In this case, it is possible that Elizabeth could have been born in January 1866 and not Baptised until the following year.
All the best.