So I have 2 sets of data, one is around 8700 values and the other is around 5100 values. Both of them are from different experimental setups, so no formulas are used. I need to find the error between those data. I have made the graphs and everything and I need to find the exact value of maximum error in the trend. The graphs do follow a trend but the error reduces eventually. I need the exact point of error. The data is currently in MS Excel. Is there any way it can be done?

To explain it further, the x-axis contains time and the y-axis contains the observed data. I need the time at which the maximum error occurs and the value of that error.