Needing a formula that I can input into excel for Concrete Maturity

I have uploaded the two sheets that I have been currently tasked with deciphering.

NOTE to OP: The conversion to C is correct, but the maturity calcs recommended by CK76 would definitely not be correct, and is actually grossly overestimating the concrete maturity.
ΔT is the data interval, not the accumulated time.
In your case, the ΔT is 1 hour. (This, however, is not the ASTM C1074 recommended ΔT, which is 0.5 hours or less.)
Also, Ta is not supposed to be an average of all the temperature values up to the current time, but an average of the temperatures across the ΔT.
Additionally, the formula recommended by CK76 is using the F values, and not the C values they had you convert in row C.

A more accurate formula would be to enter "0" in D19
In D20 enter the formula =(AVERAGE(C19:C20)-0)*1
In D21 enter the formula =(AVERAGE(C20:C21)-0)*1+F20, and then copy down.

Your Time Temperature Factor at 24 hours is approximately 608 C-Hr.

Note that the maturity method is a mix-specific test. Each mix must be calibrated as per ASTM C 1074. If you have a target maturity value, it was determined using a specific datum temperature for this mix. That could be a default value like 0C, but it is important to confirm this against the original maturity calculations used to calibrate for this specific mix design.

I hope this helps, feel free to reply here or PM me with questions. I would also recommend you find a new system. COMMAND Center is a superior alternative to the system you are using currently. It is more affordable, much easier to implement, and offers much more flexibility in terms of data collection. The sensors can be used for both temperature and maturity monitoring, so you will never have the issue you are currently experiencing where you have the "wrong" sensor for the job. Switching systems is easy, because COMMAND Center includes free software specifically designed for concrete temperature, maturity, and strength analysis, and you can use your own devices (Windows or iOS) to collect data from the sensors.

Happy to chime in! You were pretty close there. As far as the statement you mentioned, I was referring to your recommendation of:

Then in D19: Copy down.
=(Average($B$19:B19)-0)*A19

Since the the Celsius values are what we are intending to use (assuming the OP wants the TTF values in C-Hr), I believe what you meant to use was the Celsius values you had him convert in row C.

Ah, thanks. I had missed the typo.