the buyer should open trough a bank acceptable to the seller

the buyer should open trough a bank acceptable to the seller .有关信用证的一个问题
“the buyer should open trough a bank acceptable to the seller an irrevocable letter of credit payable at sight for 100% of the contract value to reach the seller by the end of April 2008 and vaild for negotiation in China until the 15th day after the date of shipment.” 其中by the end of April 2008指的是?vaild for negotiation in China until the 15th day after the date of shipment又是什么意思?
我的意思是在2008年4月底之前是信用证要开好了？还是要发过来了？