What is the difference between 于 and 以 to express "a certain time" or "a fixed date"?
As a preposition, 以(with)  and 于(at/ in) are very different.

我以三月一日返。(I take March the first come back)
It is plain wrong, the sentence make no sense.

The only way you can use 以 with a fixed date or period of time is:

In a [以 (A) 为 (B)] structure -- Example: &quot;以三月一日为限期&quot;  (take March the first as the deadline)

use '以' as preposition 'with' -- Example: &quot;以三日时间完成这工作&quot; (finish this job with three days time)



生于上海 (born in Shanghai) is correct
来信与十五日收到。There must be a typo. It should be &quot;来信于十五日收到&quot; (your letter arrived at the fifteen)

于 can be a preposition for date or location
Example:
&quot;生于上海&quot; (born in Shanghai) - location
&quot;生于一九一二年&quot; (born in 1912) - date
&quot;于上午五点死亡&quot; (died at 5:00 AM) - time