DB2中的current date問題

hanminnba

在IBM iSeries Access Windows中的Data Transfer from iSeries做數(shù)據(jù)查詢，用current date 減去生產(chǎn)日期得到安全庫存天數(shù)時，得到的數(shù)值有時與實際不符的情況，這是什么原因？AS400版本V5R4M0
select itemcode, lotcode, (current date) - (left(lotcode,4)||'-'||substr(lotcode,5,2)||'-'||substr(lotcode,7,2)) stockday from storage

Itemcode              lotcode              stockday
11200045        20101231B12        6
11200043        20110103B12        30
11200037        20110103B12        3
11200036        20101231B12        6
11200043        20110103B12        3
11200044        20101231B12        6
11200037        20110103B12        3
11200036        20101231B12        60