Calculating column values based on date match between tables

[Anonymous]

I'm attempting to perform a column calculation that will return a value based on the last date of an instrument calibration prior to a quality control check. I have two tables, Calibration and QC, which are related by instrument serial number. For each instrument, there may be multiple calibration dates and quality control checks, during which a result (Xq) is generated. The calculated column (cal check) should compare the difference between the QC check Xq and the Xq result of the last calibration prior to that check. 

Example:

Calibration

Serial	Cal Date	Cal Xq
100141	12/5/19 14:12	0.081
100141	4/12/15 11:36	0.085
100290	6/16/15 9:19	0.077
100321	1/21/19 14:44	0.082
100321	1/21/19 11:12	0.079
100670	1/2/20 11:24	0.079
100670	9/4/18 15:39	0.081

QC

Serial	QC Date	Cal Date	QC Xq	Cal Check
100141	6/15/19	4/12/2015	0.081	-4.71%
100141	10/15/19	4/12/2015	0.082	-3.53%
100141	2/15/20	12/5/2019	0.079	-2.47%
100141	6/15/20	12/5/2019	0.08	-1.23%
100290	6/15/19	6/16/2015	0.079	2.60%
100290	10/15/19	6/16/2015	0.078	1.30%
100290	2/15/20	6/16/2015	0.077	0.00%
100290	6/15/20	6/16/2015	0.077	0.00%
100321	6/15/19	1/21/2019	0.08	-2.44%
100321	10/15/19	1/21/2019	0.081	-1.22%
100321	2/15/20	1/21/2019	0.079	-3.66%
100321	6/15/20	1/21/2019	0.082	0.00%
100670	6/15/19	9/4/2018	0.079	-2.47%
100670	10/15/19	9/4/2018	0.077	-4.94%
100670	2/15/20	1/2/2020	0.082	3.80%
100670	6/15/20	1/2/2020	0.081	2.53%

Fortunately, there is a helper column in the QC table that lists when the last cal was performed. Unfortunately, it is just a date with no time stamp, whereas the Cal Date column in the Calibration table does include the time, and there may be multiple entries from the same day. If I were doing this in Excel, I would use an array to match serial number and cal date between the two tables to return the appropriate Cal Xq, and calculate the difference from there. If they are sorted newest-to-oldest in the table, it will give me the correct result. But, I'm new to Power BI and haven't found a comparable method of doing this with DAX. 

Any help would be appreciated, thank you!