The mechanism: a fixed block count, not a fixed date
Bitcoin's protocol halves the block subsidy every 210,000 blocks — not every four calendar years, though that's roughly how it works out in practice. Block 210,000 (Nov 2012), block 420,000 (Jul 2016), block 630,000 (May 2020), and block 840,000 (Apr 2024) have each cut the reward in half. The next halving fires at block 1,050,000.
Why the exact date can't be known
Miners find blocks by solving a probabilistic puzzle rather than following a scheduled clock. The network retargets mining difficulty every 2,016 blocks to aim for a 10-minute average block time, but any individual block can arrive in seconds or take hours by chance. Over 210,000 blocks the variance mostly averages out — but a sustained change in total network hash rate (more or less mining power joining or leaving) shifts the real-world date by actual days to weeks in either direction. What the countdown tracks is block height; the calendar date is only ever a projection from an assumed average block time.
What changes for miners
The block reward drops from 3.125 BTC to 1.5625 BTC per block at the next halving. Combined with transaction fees, that's the entirety of miner income for securing the network — the same issuance-side economics behind this site's Puell Multiple and Security Budget pages.
Does the halving itself move price?
Not mechanically, and not on the day it happens. The halving date is public knowledge years in advance, so any effect from reduced future issuance should already be priced in under efficient-market logic. Historically, the larger price moves have shown up in the 12–18 months following a halving rather than around the event itself — a correlation with several plausible explanations behind it, none of them a proven causal mechanism tied to the halving specifically.
For the live countdown, current epoch progress, and every past halving marked directly on the price chart, see the Halving Countdown page.