Here are two sentences.
The very first few weeks that I spent at the new company, I made a lot of mistake.
The very first few weeks when I was new to the new company, I made a lot of mistake.
Here, can I use both? the point is that incomplete clause(without object of spent) follows that, whereas complete clause follows where. Whether it's 'that' or 'where', can I use both to make this phrase an adverbial clase?
Another point I'd like to make sure is if "The very first few weeks when I was new to the company" can be a time adverbial phrase or not. the reason I ask this question is I know I can use " The last time I went to the book store, what I wanted to purchase was sold out. "
Finally, as the very fist few weeks represents duration , Do I need to use 'for' right before the very first few weeks?