Backtest start date — six months changes the result
Same asset, same length — but shift the start by six months and the result changes completely. What that 'am I buying too late?' question really asks.
Reading investment materials, a question shows up in your head at some point. "Am I buying too late?" It hits harder after a stretch of news saying the market has run up a lot. Unpacking the question once makes it lighter, though. Here's how much the start date moves the result, and why that influence is something you can't actually solve.
Why six months changes the result
Run the same asset for the same length and the result still splits. Run SPY for 5 years with $1,000 monthly DCA, and the choice between "start in January 2018" and "start in July 2018" alone can produce different final values.
The length is the same 5 years. The asset is the same SPY. The contribution amount is the same. The result still differs.
The reason is straightforward. Within those 5 years, the distribution of up moves and down moves lands differently depending on where the start date sits.
- January 2018 start → meets the late-2018 correction early on → cheaper buys, then a long climb through recovery
- July 2018 start → meets the late-2018 correction right after starting → less time to ride the recovery
Same 5 years, different positioning of the market's swings relative to the start. The start date enters the result directly.
Where the window lands — same length, different inside
Going one level deeper. A backtest calculates results within a fixed window — "from this date to that date." Same window length, different placement, different market path inside.
Picture buying rice. The same 12 months of rice from January through December versus July through next June can carry different prices. Within that year, the up-and-down moves of rice prices land differently. The start date moves the result the same way.
On the result card, numbers like CAGR (compound annual growth rate) and MDD (maximum drawdown) shift even when only the start-date slider moves. Same asset, same length, same other options. Move the start date one notch and the numbers move with it. That's start-date sensitivity.
It's a property of backtesting itself, which means you can't sidestep it. Wherever the start sits enters the result directly.
What "am I buying too late?" actually asks
Unpacking the question once shows its shape. "Am I buying too late?" actually means "Am I picking a worse start date than other people would?"
Two assumptions sit underneath.
- One assumption: "There's a better start date out there."
- Another assumption: "I'd be able to pick it."
Both assumptions are hard to verify in any meaningful way.
A good start date is usually something you can only know after seeing the result. A buyer who started in March 2020 only knows it was a good start after watching the recovery unfold. People at that same moment were also asking "am I buying too late?" Because everyone evaluates after the fact, "good start date" is a concept that gets organized in hindsight.
Holding that point makes the question feel lighter. "Am I buying too late?" is a question that needs a future answer, and no one can confirm that answer from where you stand now.
The cost of betting everything on one moment
Following that thought through, the next idea arrives naturally. "Then is picking one moment and putting everything in really a good choice?"
The start date enters the result directly, but you can't accurately pick the start date in advance. So putting all your capital in at one moment is essentially tying the entire result to one start date.
Read as a cost, it looks like this.
- Pick a good start date → good result
- Pick a bad start date → same asset, same length, bad result
The problem is that whether you'll pick well or poorly isn't something you can know now. That's the inherent cost of betting everything on one moment. The entire result rides on one moment's luck.
Recognizing that cost leads naturally to the next question. "Is there a way to spread the timing luck?"
How to use the start-date slider
In the simulation, move the start-date slider slowly once and the property settles into your hand. Same asset, same length, shift the start by 6 months, 1 year, 2 years. Watch how CAGR moves. Watch how MDD moves.
Running several start dates one after another puts three things in your head.
- Even with the same asset and same length, the range of results runs wide
- One start date's result is a single point inside that range
- Whether that point lands on the better or worse side of the range isn't something you can know in advance
Once those three points are in view, "am I buying too late?" looks different. The deeper question isn't whether one start date is good or bad — it's whether there's a way to invest that doesn't depend on the start date.
That's why the start-date slider sits right on the result screen. Don't get locked into one start. Try several.
Market timing vs time diversification
Pulling this together, two paths of thinking branch apart naturally.
- Market timing attempt → pick a good start date in advance and enter at one moment
- Time diversification → don't pick one start date, spread the entry across multiple moments
Market timing rests on the assumption that you can pick the good start date in advance. Time diversification rests on the acknowledgment that you can't.
That's the reason the recurring contribution option exists in the simulation. Spread $1,000 a month across 12 months and the entire result no longer rides on a single start moment. Twelve entries across twelve different dates make the result curve from "6 months earlier vs 6 months later" considerably flatter.
That's the decisive difference between lump sum and DCA. Lump sum ties the result to one start moment. DCA spreads the start across multiple moments, dampening the result curve. For a deeper look at lump sum versus DCA, the related article walks through that side directly — read this one for "why the start date enters the result," and that one for "how to spread the same amount across time."
Looking at it directly
Running it directly is the fastest way. Same asset, same length, same contribution amount — only the start date shifts by 6 months across three to five runs. Watch how CAGR and MDD move together, and start-date sensitivity settles into your hand.
Don't bet the result on one start moment. Shift it 6 months at a time across five runs. That's when luck and decision separate.
- This information is not investment advice.
- Past performance does not guarantee future results.
- Backtest results are simulations and may differ from actual trading outcomes.
Kistack is an information service designed to help users review market data independently and form their own judgments. These backtests are historical simulations based on public market data and do not guarantee future investment returns. Past performance is not indicative of future results. Trading costs such as fees, taxes, and slippage are not reflected in simulations. Data is provided by Kistack; decisions are made by users.
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