This occurred on our fourth day after reaching base camp, by which time we had almost gotten our bearings.
这个by和 time是如何来的，有哪些用？
which的先行词是什么？
能否把这个定语从句还原一下？
这个句子是由下面二个单句合并而来的。
This occurred on our fourth day after reaching base camp. Bythat time we had almost gotten our bearings.
指示代词that指代主句内容，that time意为主句动作发生的时间。
当需要将这二个简单句合并为一个句子时，有二种选择：并列（二个句子重要程度相等）和从属（重要程度有主次之分），所以大家可以得到一个并列句和一个主从复合句。
1 并列句，用并列连词and连接二个分句：
This occurred on our fourth day after reaching base camp, and bythat time we had almost gotten our bearings.
2 主从复合句，用从属关联词——关系代词替换指示代词：
This occurred on our fourth day after reaching base camp, by which time we had almost gotten our bearings.
