Sum values based on dates

Have what is probably either really simple or completely impossible to do.
I want to be able to dynamically adjust a value which is a product of values summed across column the length of which a based on a date value

Here's the scenario
Row 1 contains a 21 day date range starting on column B
Rows 2-50 have a dollar amount that corresponds to Row 1. Together they show a the cost/day.

Clear over on column AJ I have a date that changes based on a bunch of other formulas and calculations throughout the rest of the sheet.

My hope is that I can have Excel calculate the sum of the dollar amount up to the date that shows up in column AJ

For example:
Row 1 10/12, 10/13, 10/14.....10/31
Row 2 $100, $150, $130, $175.....$145

Cell AO2 has the date 10/25
I would like to sum the values on Row 2 from 10/12 (B2) to 10/25 only (P2).

If the date in AO2 changes to 10/23 for example (due to the rest of the things going on in the spreadsheet), I'd like the values summed to go only to 10/23 (N2) and so on.

I tried using SUMIF(B1:Y1,"<AJ2",B2:Y2) but it returns a 0.

Any help/ideas would be very appreciated. If I can make this work it will make my life 1000x easier!

Thanks in advance

Sorry, typo on my part, the cell I need to reference for the date to calculate to is AJ2 not AO2.
You NAILED IT though! I cannot thank you enough for the help!