Risk is often discussed as a single quantity, usually meaning how much a value moves. Which risk actually threatens a goal depends on when the money is needed.
Short horizons are dominated by volatility
Money required within a few years cannot absorb a decline, because there is no time for a recovery before it must be spent.
A fall of even a modest size can be permanent in effect, since the asset must be sold at whatever price prevails on the date required.
This is why short-term goals are usually funded with cash or short-dated instruments despite their lower expected return.
Long horizons are dominated by erosion
Over decades, the principal threat is that purchasing power falls faster than the holding grows. Stability of nominal value is no protection against this.
Cash held for thirty years is nominally safe and can lose a substantial share of what it buys. The risk is real but invisible in the account balance.
Assets with higher expected returns carry more short-term movement but address the erosion problem that cash cannot.
The two risks pull in opposite directions
Reducing volatility usually means reducing expected return, which increases exposure to erosion. Increasing expected return does the reverse.
There is no allocation that minimises both simultaneously, which is why horizon is the input that resolves the trade rather than a preference for safety.
Portfolios holding several goals with different horizons therefore need different treatment for each rather than a single allocation.
Horizon is rarely a single date
Retirement spending is drawn over decades rather than withdrawn at once, so part of the portfolio has a short horizon and part a very long one.
Treating the whole balance as though it were needed on the retirement date overstates the short-term risk and understates the long-term one.
Matching the shape of the holdings to the shape of the spending is what this observation implies.
Horizons can shorten without warning
A goal assumed to be distant can become immediate through job loss, illness or a change in circumstances. The allocation was appropriate for the assumption, not the outcome.
An accessible cash reserve exists partly to prevent this, by removing the need to liquidate long-horizon holdings at short notice.
The reserve is what allows the rest of the portfolio to be invested according to its intended horizon rather than a defensive one.