Duration.

Concept

The two-axis model

Duration is built on one idea: every amount you hold has two labels, not one. Get both down and the questions that used to need a spreadsheet answer themselves.

Purpose and horizon

An amount’s purpose says what it is for — rent, a holiday, a new roof. Its horizon says when you will need it: out-of-pocket, short, medium or long term. The purpose is the familiar half of budgeting. The horizon is the half most tools leave out, and it is the one that decides where the money physically sits — cash for the near term, a low-risk fund for later, shares for the long run. You already run money this way; Duration just writes it down.

Allocated and funded

Because money spread across accounts moves on its own schedule, Duration keeps two numbers where a single-account budget keeps one. Allocated is what you have decided a horizon should hold — intent. Funded is what its accounts actually hold — reality, the figure your bank statement matches. You decide on payday; the cash follows when you transfer it, so the two need not agree at every moment.

Drift, and closing it

The gap between allocated and funded is drift — not an error, just the cash not yet caught up with the decision. Duration separates it into two kinds that call for opposite responses:

  • Timing drift — intent the cash has not reached yet. A sweep of cash between your accounts closes it.
  • Market movement — gains and losses on invested money. You attribute these across your buckets when you choose; they are never swept, because the money is already there.

Keeping the two apart is the whole point. A single number that blurred them would nudge you toward the wrong move — selling a fund to cover something that was never short.

But won’t I be taxed every time I move money?

A fair question, and often the first one — if selling inside an investment account is taxed every time, doesn’t the horizon idea fall apart? The honest answer is that it depends on where you are. Tax systems treat this very differently around the world, and it’s easy to assume everyone starts from the same place when they don’t.

Some countries offer accounts where moving between holdings is free of tax, or nearly so — Sweden’s ISK, Norway’s ASK, the UK’s ISA, Canada’s TFSA, and others like them. Where you have one, Duration’s horizon model runs without friction: near-term money can sit in something steady and longer-term money in shares, and re-homing between them costs nothing at tax time. That is the setting Duration was first shaped around, so if this is you, everything here is meant to work end to end.

Where you don’t — a plain taxable account, where each sale is a taxable event — nothing breaks; the horizon axis just asks less of you. Keep your near-term horizons in cash, hold investments only where you already would, and the rest of Duration is unchanged: purpose and horizon, allocated versus funded, drift, the whole legible picture of what your money is for. Investing across horizons is something Duration supports, not something it asks of you. And it’s worth knowing that accounts like the ones above exist — for many people elsewhere they are the ordinary way to save, and they may yet appear where you are too.

Books: one picture per set of money

Some money isn’t yours alone to plan. A co-owned joint account you and a partner both fund is the clearest case — it’s real money with its own bills and its own sinking funds, and leaving it untracked would hide your household’s largest flow. But it doesn’t belong layered over your personal budget either, where the same money would be counted twice.

The answer is a Book: a separate set of accounts and buckets you own and switch between. Each Book keeps its own clean picture — its own allocated, funded and drift — and every unit of money lives in exactly one of them. Joint household money becomes a second Book, where the joint account is tracked and swept like any other; in your personal Book, your monthly contribution to it is simply a spend. The two never touch, so neither has to pretend to be the other.

This is one budget seen honestly, never two parallel plans over one pile of money. For the full mechanics — including running an ad-hoc tab with a receivable when there’s no joint account — see tracking split expenses.

One currency per Book

Each Book keeps its record in a single accounting currency, chosen when you create the Book — any currency the European Central Bank publishes a rate for. Every figure in that Book is written and read in it, and Duration follows the currency’s own conventions: two decimals where a currency has them, none where it doesn’t. It is fixed once the Book exists, because changing it would mean restating every amount you had already recorded.

Books are independent, so a second Book can keep its record in a different currency without either picture blurring. What is not supported yet is a single Book holding accounts in more than one currency at once — for that, the accounts in a Book should be denominated in the Book’s own currency.

Amounts in these guides are written as plain numbers, without a currency symbol; read them in whichever currency your Book keeps.

Where to go next

Last reviewed: July 2026.