What is the difference between "at the time" and "in the time" when they refer to a period of time?

Question

From the page about Oscar Wilde on OALD:

E1. In 1895 he was sent to prison for his homosexuality, which was illegal at the time.

And here is a chunk from my post on Lang-8. No one has changed in to at so far.

E2. This year, I received a Bachelor's degree in biology, and now I am studying for a Master's degree. I am doing the same research I did in that time.

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What I have found on the topic:

In that time suggests during a period of time: e.g. in that time I managed to undo the knot.

At that time suggests at a particular moment, or period: e.g. at that time I was fond of talking Esperanto.
_{Source: at that time / in that time -- WordReference Forums}

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To be honest, after that explanation I don't see any difference between at and in in the examples I wrote above, since both are referring to a period.

Do these uses differ from each other when referring to a period, and what is this difference?

After a little thinking: should I have written the exact years of my study to use at in E2?

Answer 1

I would certainly change your sentence to "at that time".

What I believe you are missing is that "in" implies a duration and the phrase is actually a shortened version of "within" and skipping "amount of":

[with]in that [amount of] time I managed to undo the knot.

This means:

• I was given some amount of time
• I was able to complete the task

A longer version of the sentence, to put it in context, would be something like:

I was tied up in a closet and knew the bomb would go off in two minutes. In that time I managed to undo the knot, get out of the closet and escape the building just before it exploded.

And, if you were telling the story to someone, you might include "at that time":

A: Where were you yesterday evening?
B: I was tied up in a closet at that time and a bomb was going to go off in two minutes.

Answer 2

Here's the Google ngram for "illegal in that time, illegal at that time" - the combination "illegal in that time" was not found:

As the guy at WordReference said, "at that time" takes a "snapshot image" of something: a moment or a span of time that is probably short.

It's hard to construe a sentence with "illegal in that time".. hm..

He was given 6 months to make prostitution illegal, and he managed to push through laws that made it illegal in that time. (he managed to make prostitution illegal in 6 months = in that time)

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Here's the ngram for "did at that time, did in that time":

Here again, it's easier to come up with sentences for "did at that time":

I stumbled and the documents I had carried fell on the floor. One of the sheets proudly sported the title "Horisontal Gene Transfer", reflecting the kind of work I did at that time.

And

I stumbled and the documents I had carried fell on the floor. One of the sheets proudly sported the title "Horisontal Gene Transfer". These documents were the result of 6 months of work, and this title reflected only a portion of work that I did in that time. (meaning: "that I did [with]in that [amount of] time", "that I managed to complete within 6 months")