documents must be presented within 21 days from BL date

documents must be presented within 21 days from BL date and the date
of issuance of transport documents but within credit validity

全部货运单据应自提单日后21日内提交，且运输单据的签发日需在信用证有效期内。

（注：BL=Bill of Landing）

不明白啊 = =！