The same underlying often has markets at different deadlines (e.g. 'BTC above $X by end of June' vs 'by end of July'). The later deadline should always be at least as likely as the earlier one. When the prices across these expiries don't line up logically, you buy the underpriced deadline and bet against the overpriced one. The edge is mispriced incremental probability between adjacent timeframes.
Markets on Polymarket where calendar roll arbitrage across serial markets is the natural play:
