Grammar
Prepositions of time: at, on, in, since, and by
A meeting can start at 9:30, happen on Tuesday, and take place in October. The time expressions look similar, but each preposition gives a different amount of detail. Once you sort the expression by type, the choice becomes easier.
Choose at, on, in, since, or by to show a precise time, a date, a longer period, a starting point, or a deadline.
Read and practice
Read the examples and try the written questions in this guide. Listening recordings are not available for this lesson yet.
Try the written questions
by Friday afternoon /baɪ ˈfraɪ.deɪ ˌɑːf.təˈnuːn/
The short answer
Use at with a precise point, on with a day or date, and in with a larger period. Use since to name when a situation began, and by to name the latest acceptable time.
Our train leaves at 7:15 tomorrow morning.
Why it works: At introduces one exact clock time. In would be too broad because the sentence names the minute as well as the hour.
6 more examples
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The dentist can see you at half past three.
At is used for a precise time on the clock.
Not this:The dentist can see you
in the afternoon. What changes: In gives a broad part of the day, so the appointment time is less exact. -
We have a video call on Wednesday.
On is used with a named day.
Not this:We have a video call
in December. What changes: In is used with a month, which is a longer period than one day. -
My sister moved to Bristol in 2024.
In is used with a year.
Not this:My sister moved to Bristol
on 2 June 2024. What changes: On is used when the sentence gives a full calendar date. -
I have studied English since I started secondary school.
Since introduces the starting point of an activity that continues to a later time.
Not this:I studied English
for three hoursafter school. What changes: For gives the length of the activity, while since gives its starting point. -
Please send the form by Friday afternoon.
By gives a deadline. The form can arrive earlier, but not later.
Not this:Please send the form
on Friday afternoon. What changes: On names the day of the action; it does not say that Friday is the latest possible time. -
The library is quiet at night.
At night is a common fixed time expression.
Not this:The library is busy
in the evening. What changes: In the evening follows the usual pattern for parts of the day.
Read the full guide
First, look at the noun or phrase after the blank. Is it a time on a clock, a weekday, a month, a starting point, or a deadline? That category usually gives you the preposition. Then read the whole sentence. A deadline with by and a starting point with since can both mention a date, but they answer different questions.
Use at for a point in time
Use at when the expression feels like one point on a timetable or clock. This includes times such as at 8:00, at noon, and at midnight. Some fixed phrases also use at, including at night and at the weekend in British English.
The film starts at quarter past eight.
Use in the evening for a general period. Use at 8:15 for one exact point inside that period.
Choose the right word: The shop opens ___ 9 a.m.
Use at because 9 a.m. is a precise clock time.
Use on for days and dates
Use on with weekdays, weekends when you name a particular one, and calendar dates. A day can also include a part of the day: on Monday morning and on her birthday evening. The named day is the reason for on.
The class has its speaking practice on Thursday morning.
In Thursday morning is not correct. Thursday is a named day, so it needs on.
Choose the right word: Her birthday is ___ 18 September.
Use on because 18 September is a date.
Use in for larger periods
Use in with months, years, seasons, and broad parts of the day. These expressions contain many possible points in time, so they take in. We also say in the past, in the future, and in the summer.
The school holidays begin in July.
On July 1 names one day. In July leaves the exact day open.
Choose the right word: They met for the first time ___ 2019.
Use in because 2019 is a year.
Use since for the starting point
Since tells us when a state or activity began. It often appears with the present perfect when the state continues now: I have known him since 2020. The expression after since can be a date, a time, or a clause such as since we moved house.
Amina has worked at the café since March.
For six months tells us the duration. Since March tells us where that duration began.
Choose the right word: We have been friends ___ primary school.
Use since because primary school is the starting point of the friendship.
Use by for the latest time
By sets a limit. If a manager asks for a document by 5 p.m., it may arrive at 3 p.m. or 4:45 p.m., but it must not be late. This makes by useful for instructions, plans, and promises.
You need to check in by 6 p.m.
At 6 p.m. means the action happens at that exact moment. By 6 p.m. means it happens before or at that moment.
Choose the right word: Please reply ___ the end of the week.
Use by because the sentence gives a deadline for the reply.
Before you go
Remember this
Choose the size and purpose of the time expression. Use at for an exact point, on for a day or date, and in for a larger period. Use since for a starting point that leads forward, and by for a deadline. Read the whole sentence when both a date and a deadline are possible.
Practice with the written examples
You can practice with the written questions in the Learn tab. Each answer includes an explanation.
Try the written questionsListening test not available yet
This lesson does not have the recordings needed for a listening test. Use the written questions in the guide to check your understanding.
Try the written questionsQuestions about "Prepositions of time: at, on, in, since, and by"
When do I use at for time?
Use at for a precise point in time, especially a clock time such as at half past three. It is also used in fixed expressions such as at night, at noon, and at midnight.
The dentist can see you at half past three.
Why do we say on Wednesday but in December?
Use on with a named day or date, so Wednesday takes on. Use in with a month, year, season, or broad period, so December takes in. The choice depends on the type of time expression.
The dentist can see you in the afternoon.
Do I use on or in with a full date?
Use on with a full date, for example on 2 June 2024. Use in when you mention only the month or year, such as in June or in 2024. A date identifies one day inside the larger period.
We have a video call on Wednesday.
What is the difference between since and for?
Since names the starting point of a continuing period, as in since I started secondary school. For names the length of time, as in for three hours. They answer different questions about time.
We have a video call in December.
What does by Friday mean?
By Friday means no later than Friday. The action may happen earlier, but Friday is the deadline. This differs from on Friday, which only states the day when an action happens.
My sister moved to Bristol in 2024.
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