[splithalf]

It’s such a fundamental contradiction you see it everywhere. The quote “buy low, sell high” says we should time the market. Even the classic “percentage of bonds to stocks should be your age” requires us to time the market. And if you just buy stock when you happen to have spare cash, that too is “timing the market.”

[sokoloff]

I would argue that the percentage of bonds should be your age is not timing the market in any meaningful sense of that phrase. (I also think that's too conservative an asset-allocation, but in any case "make a periodic rebalancing trade according to this preset formula" is the opposite of "time the market".)

[splithalf]

I would argue that the distinction between periodic rebalancing and timing the market is subjective. Even if you aren’t trying to rebalance based on market conditions, ie selling at a high, the downsides of market timing are still there just the same. Just because you don’t care about the timing of your trades doesn’t mean the timing doesn’t matter.

[sokoloff]

By that logic, the initial investment timing matters as well, which is technically true of course but practically not particularly useful. If you want exposure to equities, you have to buy equities at some time.