Target date funds sit at the center of most American workplace retirement plans, usually as the default option. Their defining feature is a mix that changes on a schedule.
The glide path is the product
Each fund publishes a glide path, a predetermined schedule describing what share of the portfolio sits in stocks, bonds and other assets at every point before and after the target year.
Early in the path the allocation leans heavily toward equities. As the target year nears, the fund shifts progressively toward bonds and short term instruments.
The manager executes that schedule mechanically rather than reacting to market conditions, which is what distinguishes these funds from actively managed balanced portfolios.
To and through are different designs
A fund designed to reach its most conservative mix at the target year is described as a to retirement fund, and its glide path flattens on that date.
A through retirement fund continues shifting for years or decades afterward, on the reasoning that a retiree may need decades of further growth to fund a long life.
Two funds carrying the same year in their names can therefore hold noticeably different amounts of stock on the same day, which surprises investors comparing them by name alone.
The year is a label, not a guarantee
The date in the fund name identifies the approximate year the investor expects to begin withdrawing. It says nothing about the value the account will reach.
Funds close to their target still hold market exposure and can fall in value, as investors near retirement have experienced during broad market declines.
Nothing in the structure protects a balance, and none of these products makes any promise about the amount available at any future point.
Costs stack in layered structures
Many target date funds are funds of funds, holding underlying index or active portfolios rather than individual securities directly.
The investor pays the expenses of the underlying holdings and, in some cases, an additional layer at the wrapper level, which the prospectus states explicitly.
Because these costs apply to the whole balance every year, the difference between a low cost and high cost version compounds over a working lifetime.
Holding one alongside others changes the design
A target date fund is built to be the entire portfolio, since its whole purpose is controlling the overall allocation.
Combining it with separate stock funds moves the actual allocation away from the glide path, so the schedule no longer describes what the investor holds.
That is a factual consequence of how the product is constructed, and it is worth understanding before assuming the fund is doing what its name implies.