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.

Where to go next