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.
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
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?