BOJ Policy Decisions and TONA: How to Match the Dates and Rates

To compare a Bank of Japan policy decision with TONA, match the policy's effective date to the date of the transactions, then check the release version and the weighted-average rate column. The date beside a download link may be the publication date, not the transaction date.

TONA, the Tokyo Overnight Average Rate, is Japan's uncollateralized overnight call rate. A BOJ policy statement specifies the level the Bank intends to guide that rate toward; the daily statistic reports a weighted average of covered transactions. Read the statement and the data file together rather than substituting one number for the other. BOJ terminology · BOJ explanation of the rate

Start with four dates

Keep these dates separate when you open a BOJ statement and a daily rate file:

The reading order is decision → effective date → observation date → publication and version. It is a sequence of checks, not a claim that all four dates are different.

For example, the BOJ's statement dated September 18, 2026 specified September 24, 2026 as the start date for its revised guidance. When reading a rate for a day between those dates, do not apply the new guidance merely because the decision had already been announced. Check which statement applied to the transaction day. BOJ statement, page 1 and footnote 1

Next, read the date inside the daily release title, not just the date beside the link. The BOJ's daily index, checked on October 25, 2026, showed this pair:

Those two releases do not describe the same transaction day. To compare provisional and final results, first find versions with the same observation date. This example uses the release labels only; it does not give an observed TONA value. BOJ daily data index

Choose the overnight weighted-average rate

In the BOJ data, identify uncollateralized overnight call rate, then use its weighted average, expressed as a percentage. Do not pick the highest or lowest rate, and do not mistake transaction volume or outstanding balances for a rate.

The BOJ's methodology covers specified transactions reported by its information providers. For uncollateralized call rates, this means brokered transactions through those providers. It is not a census of every loan or every direct transaction in Japan's money market. The rate tables also distinguish overnight transactions from other maturities. BOJ statistical methodology, pages 1–2

“Overnight” means the next business day

For the O/N category used here, the transaction is agreed and the funds are transferred on the same day; repayment falls on the next business day. “Tomorrow” is therefore too loose a definition.

For example, if Friday and Monday are business days and there is no intervening business day, a Friday O/N transaction matures on Monday. If Monday is a holiday, the next business day is later. Do not switch to tomorrow/next (T/N) or spot/next (S/N) merely because a weekend intervenes: those categories have different settlement timing. BOJ statistical methodology, page 1

How volume weighting changes the number

The following figures are entirely hypothetical, not current or historical BOJ rates. Suppose the policy target on a hypothetical day is around 0.500%, and the covered transactions consist of only these two groups:

The weighted average is:

(0.450 × 10 + 0.470 × 30) ÷ (10 + 30) = 0.465%

The larger group receives three times the weight of the smaller group. Simply averaging the two rates would give 0.460%, which ignores the different transaction amounts. The BOJ's published methodology weights rates by transaction volume and rounds the percentage rate to three decimal places. BOJ statistical methodology, page 2

In this fictional example, the weighted average minus the target is:

0.465% − 0.500% = −0.035 percentage points = −3.5 basis points

One basis point is 0.01 percentage point. The calculation shows a difference between two hypothetical numbers; it does not explain the cause of a real-world difference or establish that policy has changed. It also does not calculate interest payments, compounding or day-count adjustments.

Check provisional, final and corrected results

The BOJ's English daily data page calls the first release provisional results; you may also see it described as preliminary. Keep that version label with the observation date whenever you save a value.

As checked on October 25, 2026, the BOJ listed the following approximate publication times, in Japan Standard Time:

Use the current BOJ schedule when checking a later release. These are scheduled times, not a promise that every file will always appear on time. BOJ daily data and release schedule

A final result can still be corrected. The BOJ's usage notes allow for post-publication corrections and changes or interruptions to publication. Check its correction notices rather than treating “final” as “unchangeable.” BOJ notes on using the rate

To make the comparison easy to check later, save a short record:

• Policy: statement URL, decision date, effective date and stated target.
• Observation: transaction date, uncollateralized O/N weighted-average column and percentage unit.
• Release: file URL, publication date, provisional/final label and the time you accessed it.
• Corrections: any applicable notice and the version of the value you used.

If you need the updated value, use the final result and check for later corrections. If you are reconstructing what was available at an earlier time, retain that earlier release and label it accordingly. Do not silently replace one version with another.

Frequently asked questions

Does a different TONA number mean the BOJ raised rates?

Not by itself. Check the dated policy statement and its effective date. A change in the observed weighted-average rate alone does not establish a new policy decision.

Are provisional and final results published on the same day about the same transactions?

Not necessarily. Under the listed schedule, the morning final release covers the previous business day, while the later provisional release covers the business day concerned. Match the observation date before comparing versions.

Can I use the policy target when a daily rate is missing?

No. The target is not a substitute for the observed weighted average. Treat the value as unavailable until you have the relevant BOJ data, with its date and version.

The practical rule

Before comparing any two figures, ask: Which policy was in force, which transaction day does this rate cover, and which column and release version am I reading? That check prevents a publication-date mismatch or a provisional/final mix-up from becoming a misleading comparison.

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Sources and scope

Source verification: October 25, 2026. The article uses saved BOJ evidence for the dated examples and methodology; the numerical rate calculation is hypothetical, not an observed TONA value.

1. BOJ terminology
2. BOJ explanation of the rate
3. BOJ statement, page 1 and footnote 1
4. BOJ daily data index
5. BOJ statistical methodology, pages 1–2
6. BOJ notes on using the rate

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