What are Prepositions of Time?
Prepositions of time are small words — at, in, on, for, since, during, by, until — that tell us when something happens, how long it lasts, or the boundaries of a time period. Without them, sentences about time would be incomplete or ambiguous.
The three you will use most often are at, in, and on. Each belongs to a different size of time: at for exact moments, in for longer periods, on for specific days and dates. Once you learn which type of time each one belongs to, choosing the right preposition becomes automatic.
Beyond the big three, a second group — for, since, during, by, until, from…to — handles duration, deadlines, and time ranges. This page covers all of them.
The Big Three: AT, IN, ON
The single most useful rule: match the preposition to the size of the time expression.
AT + exact time · IN + period · ON + day / dateOverview
| Preposition | Used with | Examples |
|---|---|---|
| AT | Clock times, mealtimes, fixed points | at 7 o'clock · at noon · at midnight · at night · at the weekend |
| IN | Parts of the day, months, seasons, years, centuries | in the morning · in July · in summer · in 2019 · in the 20th century |
| ON | Days of the week, specific dates, special days | on Monday · on 12 March · on Christmas Day · on her birthday |
When to Use Each
AT — exact times and fixed points
We use at for precise clock times, mealtimes, and a small group of fixed expressions. Think of at as pointing to a specific dot on the timeline.
- The train leaves at 6:45.
- I always have coffee at breakfast.
- She usually goes to bed at midnight.
- We arrived at night, so we couldn't see anything.
- Are you free at the weekend? (British English)
IN — longer periods: parts of the day, months, seasons, years
We use in for time containers that are bigger than a single day. Think of it as being inside a period of time.
- I usually exercise in the morning.
- Her birthday is in March.
- We always go hiking in autumn.
- He was born in 1998.
- This style was popular in the 1960s.
ON — days and dates
We use on for any specific day — whether a day of the week, a calendar date, or a named special day. Think of the event sitting on top of that particular day.
- I'll call you on Monday.
- The conference starts on 14 September.
- What are you doing on New Year's Day?
- She's coming on her birthday.
- I was born on a Tuesday.
FOR and SINCE — duration and starting point
For says how long something lasts. Since says when something started (and is still true now). They are often confused but answer different questions: for answers "how long?"; since answers "from when?".
- I've lived here for three years.
- I've lived here since 2021.
- She studied for two hours before the exam.
- He hasn't eaten since breakfast.
DURING, BY, UNTIL, and FROM…TO
These prepositions handle specific time relationships that at, in, and on cannot express: what happened throughout a period (during), a deadline (by), an end-point (until/till), and a time range (from…to).
- I fell asleep during the film. (during = throughout a period)
- Please finish this by Friday. (by = no later than)
- I'll wait until 6 o'clock. (until/till = up to that time)
- The office is open from 9 to 5. (from…to = start and end)
Key Expressions
FOR vs SINCE
This is the most commonly confused pair. Both relate to time that has passed, but they answer different questions.
FOR — how long?
I have worked here for five years.
Five years = a length of time. For answers "how long?" — it measures a duration.
SINCE — from when?
I have worked here since 2019.
2019 = a starting point. Since answers "from when?" — it names the moment it began.
FOR + duration
She waited for an hour.
An hour is a measure of time — you can use for in any tense (past, present, future).
SINCE + point in time
She has been waiting since noon.
Noon is a clock time — a starting point. Since almost always appears with perfect tenses.
Memory trick: For = a span of time you could draw as a line segment. Since = a dot on the timeline — the starting point. "I've known her for three years / since 2021."
Common Mistakes
Using IN instead of ON for days
✗ I'll see you in Monday. / The party is in Saturday.
✓ I'll see you on Monday. / The party is on Saturday.
Days of the week always take on — not in. Think: the event sits on top of that specific day. Months and years are inside a period, so they use in. Days are single points, so they use on.
Using AT instead of IN for years and months
✗ She was born at 1995. / We go on holiday at August.
✓ She was born in 1995. / We go on holiday in August.
Years and months are periods, not points — use in. At is for exact clock times and a small set of fixed expressions.
Using IN instead of AT for clock times
✗ I'll meet you in 3 o'clock. / The film starts in 8 p.m.
✓ I'll meet you at 3 o'clock. / The film starts at 8 p.m.
Clock times use at because they are exact points in time, not periods. Remember: at for exact moments, in for longer periods.
Confusing FOR and SINCE
✗ I've lived here since three years. / She hasn't eaten for lunchtime.
✓ I've lived here for three years. / She hasn't eaten since lunchtime.
For needs a duration — a length of time (three years, two hours, a week). Since needs a starting point — a moment or event (lunchtime, Monday, 2019).
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