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Table 2.3.1 provides 1 and 5 day thetas for underlying prices ranging from $99.50 to $99.90 and assumes a strike price of $100 and therefore applies to Fig 2.2.1. The theta for the $99.70 profile with 5 days left to expiry is –6.5057. This value of theta defines how much the upbet will decline in value over one year at the current rate of decay. To gauge how much the upbet will lose in time decay over 1-day divide the theta by 365 so the rough estimate of one-day decay at 5 days would be –6.5057 / 365 = –0.017824. But remember, by convention binary prices are multiplied by 100 to establish trading prices within a range of 0 – 100, so likewise we need to multiply the theta by 100 to get comparable decay. In effect the time decay over 1 day of an upbet with 5 days to expiry is –0.017824 ¥ 100 = –1.7824 points. In fact the upbet with 5.5 and 4.5 days to expiry is worth 28.2877 and 26.2938 respectively, a decay of –1.9939, so it can be argued that a 5-day theta of –1.7824 is a reasonably accurate measure.