When do I use 届时 vs 到时?

Use 届时 in formal writing, official announcements, and scheduled events. Use 到时 in everyday conversation, informal writing, and any situation where you want a neutral, colloquial tone. The choice depends on the register of the context.

Can I use 到时 in a business email?

It depends on the formality. In a very formal business email (e.g., to a client or in a contract), use 届时时. In a casual internal email or with close colleagues, 到时 is fine and even preferred for a friendly tone.

Is 届时时 only used for future events?

Yes, 届时 always refers to a future scheduled time. It cannot be used for past events or unscheduled future moments. For past or unscheduled, use 到时 or 当时 (dāngshí 'at that time in the past').

Are 届时 and 到时 grammatically interchangeable?

They are both adverbs of time that usually appear at the beginning of a clause, but their usage is not free due to register. Grammatically they are similar, but pragmatically they serve different contexts. Replacing one with the other will change the tone from formal to casual or vice versa.

Easily confusedHSK 6

届时 vs 到时 (jièshí vs dàoshí): formal vs colloquial 'when the time comes'

Both 届时 and 到时 mean 'when the time comes' or 'at that time,' but they differ sharply in register: 届时 is a formal, literary term used in scheduled events, official notices, or speeches, while 到时 is colloquial and common in everyday conversation. Learners often confuse them because of the shared meaning, but using the wrong one can sound awkward or inappropriate.

The core distinction between 届时 (jièshí) and 到时 (dàoshí) lies in formality and context. 届时 is a formal, written word that often implies a scheduled or predetermined time, appearing in announcements, invitations, legal documents, and formal speeches. 到时 is the neutral, everyday term used in speech and informal writing to refer to a future point when something happens. While both can be translated as 'when the time comes,' 届时 carries a sense of expectation and planning, whereas 到时 is more flexible and can refer to any future occasion. Misusing them can mark a speaker as either too stiff or too casual.

When to use each

届时jièshí

at the scheduled time (formal)

Use 届时 in formal writing, official notices, speeches, or any context where a scheduled or predetermined time is emphasized. It is common in invitations, meeting agendas, legal terms, and academic announcements. Avoid 届时 in casual conversation; it will sound unnatural.

届时 often implies that the time has been set in advance and carries a tone of expectation or obligation (e.g., 'all participants are expected to attend'). It cannot be used to refer to an unspecified future moment that was not planned.

到时dàoshí

when the time comes (colloquial)

Use 到时 in everyday speech, informal writing, and any context where you refer to a future time without special formality. It can refer to both scheduled and unscheduled events, and it is the natural choice for conversations about plans, predictions, or hypothetical situations.

到时 is less specific than 届时; it does not necessarily imply a fixed schedule. It can also be used in some semi-formal writing (e.g., emails to colleagues), but its primary home is colloquial speech.

At a glance

	届时	到时
Register	Formal / literary	Colloquial / informal
Typical context	Official meetings, invitations, legal docs, speeches	Everyday conversation, texting, casual writing
Temporal implication	Usually implies a predetermined / scheduled time	Can refer to any future moment (scheduled or not)
Can it be used in speech?	Rarely; only in very formal speeches	Yes, default choice in spoken Chinese
Negation (not used)	*不届时 does not exist; use 届时不会… (formal)	不加到时不 common; use 到时候不… (colloquial)

Examples

届时
请各位届时准时参加。
Qǐng gè wèi jièshí zhǔnshí cānjiā.
Please attend on time at the scheduled time.
Typical formal invitation or announcement; using 到时 here would sound too casual.
届时
会议将于下周一举行，届时请携带身份证。
Huìyì jiāng yú xià zhōu yī jǔxíng, jièshí qǐng xiédài shēnfènzhèng.
The meeting will be held next Monday; at that time, please bring your ID.
Formal notice; 到时 would be inappropriate for written instructions.
到时
我们先吃饭，到时再讨论吧。
Wǒmen xiān chī fàn, dàoshí zài tǎolùn ba.
Let's eat first, and discuss it when the time comes.
Natural colloquial usage for a casual plan.
到时
如果下雨，到时活动就取消。
Rúguǒ xià yǔ, dàoshí huódòng jiù qǔxiāo.
If it rains, the event will be canceled at that time.
Hypothetical future situation; 届时 would sound oddly formal here.
到时
✗请各位到时准时参加。（正式通知）
✗ Qǐng gè wèi dàoshí zhǔnshí cānjiā. (zhèngshì tōngzhī)
✗ Please attend on time when the time comes. (formal notice)
This is a common error: using 到时 in a formal written instruction where 届时 is required.
届时
届时我们会通知你结果。
Jièshí wǒmen huì tōngzhī nǐ jiéguǒ.
We will inform you of the result at the scheduled time.
Acceptable in a formal email or letter; in casual conversation, 到时 would be more natural.

Common mistakes

Using 到时 in formal written instructions or invitations — should use 届时.
Using 届时 in casual conversation with friends — sounds stiff and unnatural; use 到时.
Assuming both are interchangeable in all contexts — they are not because of the register difference.
Writing 到时 in a legal document or official contract — always use 届时.
Negating 届时 as 不届时 (ungrammatical); instead use '届时不会…' or rephrase.

FAQ

When do I use 届时 vs 到时?: Use 届时 in formal writing, official announcements, and scheduled events. Use 到时 in everyday conversation, informal writing, and any situation where you want a neutral, colloquial tone. The choice depends on the register of the context.
Can I use 到时 in a business email?: It depends on the formality. In a very formal business email (e.g., to a client or in a contract), use 届时时. In a casual internal email or with close colleagues, 到时 is fine and even preferred for a friendly tone.
Is 届时时 only used for future events?: Yes, 届时 always refers to a future scheduled time. It cannot be used for past events or unscheduled future moments. For past or unscheduled, use 到时 or 当时 (dāngshí 'at that time in the past').
Are 届时 and 到时 grammatically interchangeable?: They are both adverbs of time that usually appear at the beginning of a clause, but their usage is not free due to register. Grammatically they are similar, but pragmatically they serve different contexts. Replacing one with the other will change the tone from formal to casual or vice versa.