BC/BCE dates

Capture BC (or BCE) and AD (or CE) dates in ISO 8601 date format.

ISO 8601 uses a year 0000 for the year 1 BC/BCE. For earlier years the ISO year value is negative (see table below).

ISO year transition from BC (BCE) to AD (CE)

Year as string	5 BC	4 BC	3 BC	2 BC	1 BC	1 AD	2 AD	3 AD	4 AD	5 AD
ISO value	`-0004`	`-0003`	`-0002`	`-0001`	`0000`	`0001`	`0002`	`0003`	`0004`	`0005`

Dates earlier than 9999 BC (or BCE) contain a year which contains five digits or more. This should be represented accurately within the ISO date, and should be negative (see table below for examples).

Examples:

Sample content	Format	ISO date
570 CE	`YYYY`	`0570`
1300 (AD)	`YYYY`	`1300`
February 1984	`YYYY-MM`	`1984-02`
1 February 1984	`YYYY-MM-DD`	`1984-02-01`
1300 BCE	`YYYY`	`-1299`
605 BC	`YYYY`	`-0604`
September 168 BC	`YYYY-MM`	`-0167-09`
18 October 63 BCE	`YYYY-MM-DD`	`-0062-10-18`
February 20th 13030 BCE	`-YYYYY-MM-DD`	`-13029-02-20`
130,100 BCE	`-YYYYYY`	`-130099`