The very first few weeks (that / when) I spent~~,

Question

Here are two sentences.

1. The very first few weeks that I spent at the new company, I made a lot of mistake.

2. 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?

Answer 1

The first sentence is fine. The second sentence requires a comma after "weeks" in order to be correct:

The very first few weeks, when I was new to the company, I made a lot of mistakes.

The way I would probably say this (that sounds the most natural to me) is

My first few weeks at the new company, I made a lot of mistakes.