The Difference Between “by” And “until”

WRONG Write the report until 5 pm. RIGHT Write the report by 5 pm. 

Write the report until 5 pm means “Start writing the report now, continue writing it until 5 pm, then stop writing it regardless of whether it is finished or not.”

Write the report by 5 pm means “Make sure that at 5 pm the report is written and finished”.

As a preposition of time “by” means “on or before”. E.g. We will send you the documents by Friday.

• Use “by” when you refer to a deadline.

“Until” means “up to a particular time”. E.g. We have until Friday to send out the documents.

• Use “until” when you refer to the period of time before a deadline.

“By” is used to describe an action or event that happens on or before a particular moment. E.g. We expect to know the judgment by the end of November.

“Until” is used to describe a state or situation in the period of time up to a particular moment. E.g. The court proceedings are expected to last until November.

You may think of “until” as referring to the moment a state or situation changes. E.g. Comments on the draft may be submitted until 30 April 2012.

Here’s a summary:

1.         I can do it by 5 pm.             Now   → do it and finish it →   5 pm (it will be finished)

2.         I can do it until 5 pm.             Now   →       do it       →   5 pm (I’ll stop even if I haven’t finished)

3.         I can’t do it by 5 pm.             Now   → do it / not do it →   5 pm (either way it won’t be finished)

4.         I can’t do it until 5 pm.             Now   →  can’t do it  →   5 pm (I’ll start)

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