Variable cell reference based on minimum difference

Hello,

I have a monitoring system that records a data point with a date/time stamp several times a day at random intervals. For each reading I want to calculate the change compared to the first reading that was more than 24 hours ago, which could be anywhere from 1 to 20 rows above the current one. Hence with the timestamp in col A and the value in col B, the formula in col C, for example cell C20, needs to read something like =B20-Bxyz, where xyz is the row number of the first reading that is more than 24 hours, i.e the first row xyz where A20-Axyz >1.

Can this be done with worksheet formulas?

Thanks for any help.

Harald