Original: It is the main reason for milk's extended shelf life. High Temperature Short Time (HTST) pasteurised milk typically has a refrigerated shelf life of two to three weeks, whereas ultra pasteurised milk can last much longer, sometimes two to three months.
Simplified: HTST pasteurized milk normally has a refrigerated shelf life of two or three weeks, but ultra pasteurized milk can last much longer when refrigerated, sometimes two or three months.