This is a statement as much as a question because I solved my issue. However, others may encounter something similar, so I will go ahead and post this.
I was looking for a DatePart equivalent for field calculator in ArcGIS Pro (e.g. in ArcMap could use something like DatePart("YYYY", [datefield])).
I tried using SQL: EXTRACT(YEAR FROM "datefield"), which works for a query but apparently not for a field calculation. Anyone know why this may be?
Finally, I was able to use Python 3: !datefield!.strftime('%Y') (I discovered this is also case sensitive ('%y') only returns year without century.
The documentation on this seems to be poor (or I am not very good at finding it). Hopefully, this will help others who have a similar problem. This resource also proved to be very useful.