Option time: Trading or Calendar

Gear menu → Settings → Trading → Option time. Calendar charges every hour the same. Trading (the default) weights the hours by how much variance the market prices into them, measured from index option quotes:

• A weekday overnight (close to the next open) counts for about half a session on SPX (0.48) and RUT (0.50).
• A weekend day counts for about 0.23 (SPX) and 0.19 (RUT) of a session. A weekday holiday counts for about nothing.
• An early-close session counts for about 0.37 (SPX) and 0.41 (RUT) of a normal one.
• Inside a session, decay is U-shaped: on SPX the first hour carries about 30% of the day and the last hour about 13%. Scheduled FOMC decision days have their own, later-weighted shape.

So Friday’s close to Monday’s open costs about one session’s worth of decay on SPX, not three days’. Index products share these fits (the S&P and Nasdaq indexes and their ETFs use SPX’s, the Russell 2000 and IWM use RUT’s). Single stocks use a flat fallback of 0.3 for an overnight and 0.4 for a weekend day or holiday, because earnings and news land overnight and their nights are heavier. The measured numbers come from data through spring 2026.

Note

The measured time weights cover the S&P, Nasdaq and Russell families only. Other symbols, such as DJX and VIX, use the flat fallback. Futures options always use calendar time.

The weights only reshape time between now and each close. Each leg keeps its total variance, the price now does not change, and implied volatilities are still shown on calendar time.

T+ lines

With Trading time, T+1 is the next trading day’s close, T+2 the day after, and so on, so the line count is in sessions. Near a same-day expiration the Time field reads hours, and a same-day trade after the close has no T+ lines. With Calendar, T+ is in days.

If your theta is smaller than your broker’s tonight, that is by design under Trading time; see The risk graph doesn’t match my broker.