I think you can find the information here at this forum. If not through googling with keywords such as "excel array", "excel sum array" etc
To be honest, I'm not an excel expert, thus won't be able to give a clear explanation. But I'll try my best.
As I mentioned earlier, this uses the array function, for all functions in it. SUM function, as it suggest, will sum everything that the value of IF gives.
The IF function, looks at the dates on Sheet2 and 'see' if it's the date of the one that you're interested in. If it's true, then it'll calculate, if not, zero value.
The calculate value of the true value from the IF statement, returns {1, 1, 0, 0, etc} Thus, when combined, with the sum, i.e. {1, 1}, gives us 2
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