From the current documentation:
13.28 0x1B ManufacturerDate()
NOTE: ManufacturerDate() value in the following format: Day + Month*32 + (Year–1980)*256
It is actually *512. I verified the working mechanism, and its written 512 in SMBUS1.1:
5.1.26. ManufactureDate() (0x1b) Description: This function returns the date the cell pack was manufactured in a packed integer. The date is packed in the following fashion: (year-1980) * 512 + month * 32 + day.