about time usage
1 at sometime, in Chinese we usually put time before sentence or verb, like "一九九零年我把这张画挂在那儿。" or " 我一九九零年把这张画挂在那儿“ 。 在 most of the time add a place, especially in sentence with 把。when 在 follow a time, most of the time, it is emphasis, like " 事情发生在一九九零年”
2 for some period of time, in Chinese add the time after 了, like 我把这张画挂了两个小时。(the picture hung for two hours) 我挂了两个小时的画。(it took me two hours to hang this picture)我看了两个小时电影。So no 在 will use here, and the time is after 了,before object.