'By' and 'until' are both used to indicate the end point of a state or action. However, they are often confused in academic writing.
'Until' signifies that something carries on for a time and then changes. For example, 'I was employed until today; my boss fired me this lunchtime' or 'There was peace in Europe until last month when the war in Ukraine began'.
On the other hand, 'by' indicates that something happens at or before a particular time, which is often some kind of deadline. For example, 'I need to find a job by the first of May, otherwise I am toast!' or 'Putin wants to achieve some kind of victory by 9 May, an important date in the Russian calendar'.
Problems in the use of these two terms often come up in the introduction, where writers give background information that involves describing changes over time. Take the following sentences, for example:
'In the UK, COVID-19 lockdowns lasted until July 2021. Since then, most restrictions have been lifted. However, the disease itself is likely to last until the end of time.'
'By the end of 2021, over 5 million deaths from COVID-19 had been recorded, and by March of this year, that figure had risen to over 6 million.'
In the first sentence, there is a clear change in circumstances, so it makes sense to use 'until' to explain that lockdowns were going on up to July 2021, after which restrictions were no longer in force.
In the second sentence, where many writers would be tempted to use 'until', there is no suggestion of a change in circumstances. We are just interested in what happened on or before a point on a scale, so we use 'by'.
So, will Boris Johnson resign by Friday, or will he last until the next election?
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