Sales Report - SUM IF based on multiple criteria

Hello,
I'm trying to solve an excel problem where I have a units tab and I would like to create a formula where on a different sheet I can select a date (from a list) and have the sheet auto calculate what the sales units were based on the date that was picked in the list.. The problem I have is that the dates are the column headers in my sheet. The sheet I actually use is much larger but I have attached just a snipped of some of the information.

For example (using the attachment)
On a Master Sheet I would like to be able to select a date in B2 and have the formulas in cells B4-B10 use that date to pull in the sum of units in the "Units" tab.

Any help would be very appreciated. Thanks!

Formula:

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and copy down.

Thank you so much. Worked like a charm.

You're Welcome. Please mark as solved.