Book of Mormon Seven-day Week

Mosiah 13:8; “But the seventh day, the sabbath of the Lord thy God, thou shalt not do any work, thou, nor thy son, nor thy daughter, thy man-servant, nor thy maid-servant, nor thy cattle, nor thy stranger that is within thy gates”.

2 Corinthians 11:4; “For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him.”

The 7 day week wasn’t known in BoM times and didn’t come about until the Gregorian Calendar was introduced in 1582. The Gregorian Calendar, aka Christian Calendar, was an upgrade or refinement, of the Julian Calendar which came into existence at the Nicean Council in 325 AD. 

Why were the Nephites talking about this if it hadn’t been invented yet?  This passage in Mosiah was supposedly written in BC 148, making it 1,730 years off.