648 subject added entry--chronological term

[CECSpecialistI]

https://github.com/uwlib-cams/MARC2RDA/blob/main/Working%20Documents/6XX.csv

[cspayne]

@pennylenger Do $y and $z map to timespan and place for this field as well?

[pennylenger]

I didn't map them because I think x, y, z, v are strange for 647 and 648. But we can ask.