Two different ideas under one heading
By the time sets a point by which a result or situation already exists: by the time we arrive, they will be waiting. Until sets the endpoint of duration: we will wait until they arrive. First decide whether you mean a state at a particular moment or duration up to it.
Compare a completed result with duration: By six, I'll have finished means the result will already be complete by six; I'll work until six means the work continues up to six. In the first, readiness at the point matters; in the second, the endpoint of the work does.
Both expressions normally introduce a whole clause (subject + predicate), not just a time label. A bare time expression without its own subject and predicate takes either the preposition by (a deadline, without duration) or until as a preposition (duration without a clause). The final section explains this further.
By the time the bakery opens, the first customers will already be waiting outside.
By the time the bakery opens, the first customers will already be waiting outside.
The opening of the bakery is the reference point by which the queue has already formed.
We'll keep the oven on until the bread turns golden.
We'll keep the oven on until the bread turns golden.
The endpoint of duration: the oven runs continuously up to this point and then stops.
Until: duration up to an endpoint
Until and till name the time limit of an action or state. Up to that limit, one action may continue or several may repeat: I kept calling until she answered. The action need not be continuous without pauses.
Not…until means that an action will not occur before a particular limit. I won't sign until my lawyer reads it does not guarantee signing immediately after the reading: it establishes a necessary prior condition.
Until does not mark completion BEFORE a particular point; for that, use by or by the time. Confusing “continue up to X” with “be done by X” is the main source of errors in this topic; sections 3 and 6 explain this further.
The kids can watch cartoons until dinner is ready.
The kids can watch cartoons until dinner is ready.
Permission to watch lasts until dinner is ready. This does not claim that the children will watch without any pauses.
I won't sign the contract until my lawyer reads it.
I won't sign the contract until my lawyer reads it.
Signing will not happen before the lawyer has read the contract. The precise time of subsequent signing is not specified.
She stayed at her desk until midnight.
She stayed at her desk until midnight.
Until is a preposition before the noun phrase midnight: an endpoint without a separate clause.
By the time: a reference point and its result
By the time names a moment by which a result or particular situation already exists. In this pattern, the time is followed by a clause with a subject and predicate: by the time we arrive.
The Future Perfect or Past Perfect is often used for a completed result. But a state or process is also possible: By the time we arrive, it will be dark; By the time we arrive, they will be waiting. A perfect form is not required after every by the time.
By the time does not itself determine the duration of a preceding action. Duration can be stated separately: By the time we arrive, they will have been waiting for an hour. Before a time label alone, use by: by tomorrow, not by the time tomorrow.
By the time the auditors arrive, the accountant will have filed every report.
By the time the auditors arrive, the accountant will have filed every report.
The Future Perfect in the main clause shows that filing the reports will be complete by the auditors' arrival.
By the time she noticed the leak, water had already reached the hallway.
By the time she noticed the leak, water had already reached the hallway.
The Past Perfect: the event in the main clause happened before the moment in the time clause.
Future time: a present form, not will
With both connectors, when referring to the future, the verb in the time clause takes the Present Simple, or the Present Perfect where appropriate, rather than will for ordinary future meaning. The main clause already carries the future meaning; the time clause simply names the reference point.
A rare exception lies outside the target of this rule: will can occur in a time clause when it expresses a person's willingness or insistence (“if you really want to…”), rather than simple future time. These uses are not practised below.
This restriction works alongside ordinary subject agreement: third-person singular takes -s (she finishes, he arrives), not the bare form and not will + base.
by the time / until + Present Simple, ... will (have) ...
By the time the meeting finishes, I will have sent the updated slides.
By the time the meeting finishes, I will have sent the updated slides.
Finishes is a present form in the clause, although the meaning is future; there is no will alongside the clause's verb.
I'll keep refreshing the page until the tickets go on sale.
I'll keep refreshing the page until the tickets go on sale.
Go is a present form after until; the same restriction applies here.
The past mirror: the same ideas in the past
For a completed past result, the typical pattern is by the time + Past Simple, … Past Perfect. An ordinary state or process is also possible: By the time we arrived, it was dark. The intended meaning determines the main clause's tense.
The past version of until simply expresses duration up to a past point, after which the action stopped. The Past Simple is usually enough without perfect aspect, because this concerns a completed period rather than prior completion.
Didn't…until means that the action did not occur before the named event; an ordinary past narrative then implies that it did occur. This is not a precise measurement of zero delay between events.
By the time the ambulance arrived, the paramedics had already been briefed by phone.
By the time the ambulance arrived, the paramedics had already been briefed by phone.
Had been briefed is the Past Perfect: the action was complete before the ambulance arrived.
He didn't apologise until his sister explained what had happened.
He didn't apologise until his sister explained what had happened.
The apology happened only after the sister's explanation, not necessarily in the same second.
Usage boundaries: until as a preposition and its difference from by
Until can stand directly before a noun or time expression without its own clause: until Friday, until the end of the tour. By the time cannot do this in the pattern taught here: it needs its own subject and predicate.
Compare until and by before a bare time expression: by + time sets the latest point by which an action is complete (a deadline, without duration); until + time means that something continues up to that point. Confusing the deadline “no later than Friday” with until changes the meaning to “continues up to Friday”. Choose the English expression according to the intended relationship.
The distinction: by the time = a clause + reference point/result; until = a clause or noun + duration/endpoint; by = a bare deadline without a clause or duration.
Please send the invoice by Thursday.
Please send the invoice by Thursday.
By sets the latest deadline, not a requirement to keep sending the invoice continuously until Thursday. For “not earlier than Thursday”, you can say Don't send it until Thursday.
The museum stays open until nine, but by six the ticket office has closed.
The museum stays open until nine, but by six the ticket office has closed.
Until nine expresses duration; by six sets the deadline by which the ticket office has closed.
Common mistakes
Incorrect:By the time you will finish the report, I will have left the office.
Correct:By the time you finish the report, I will have left the office.
In the time clause after by the time, express the future with a present form; will is unnecessary here because will in the main clause already carries the future meaning.
Incorrect:Wait here until I will call you.
Correct:Wait here until I call you.
The same restriction applies after until: the clause names the moment with a present form, not will.
Incorrect:By the time tomorrow, the shop will have closed.
Correct:By tomorrow, the shop will have closed.
By the time introduces a subordinate clause with a subject and predicate; before a bare time expression such as tomorrow, use the preposition by without the time.
Incorrect:By the time she arrive, I will have already left.
Correct:By the time she arrives, I will have already left.
The verb in the time clause agrees with its subject according to the ordinary rules: she requires arrives (+ -s), not the bare form arrive.
Incorrect:By the time you call, I will have leave.
Correct:By the time you call, I will have left.
After will have, use the third form of the verb (past participle): left, not the base form leave.
Incorrect:By time you finish, I will have gone.
Correct:By the time you finish, I will have gone.
By the time is a fixed phrase with obligatory the; omitting the article makes this construction grammatically incomplete.