Hi Danielle, 

Interesting question!

To me in English as in French,  although they are similar there is a slight difference between 'd'ici ...' 'by a certain time' and 'avant ...' 'before a certain time' -

1. "By a certain time" means at or before a specific time. It implies that the action or event should be completed no later than the mentioned time.

For example, "Please submit your assignment by 5 PM on Friday" means that the assignment should be submitted at or before 5 PM on Friday.

2. "Before a certain time" means earlier than a specific time. It does not include the mentioned time itself.

For example, "Please submit your assignment before 5 PM on Friday" means that the assignment should be submitted any time prior to 5 PM on Friday, but not at 5 PM.

In summary, "by a certain time" includes the specific time mentioned, while "before a certain time" does not include the specific time mentioned.

Hope this helps!