Sum of rows in array with condition

[JT99]

Hello!
My data looks like this:
Acct x1 x2 ...x24 date1 date2 ... date24
qaz 12 15 ...45 1jan19 1feb19 ... 1dec20
wsx 60 41 10aug18 10sep18

Basically, I have monthly payments x1, x2 up to x24 that a customer needs to pay on date1,date2 up to date24. Some accounts will have up to x12 and date12(12 months and not 24).
How do I get the sum of all payments(x1,x2,..,) where the corresponding date is less than a particular date for each account?
Any advice will be appreciated

[Patrick]

Transpose your data into a narrow format with columns Acct Amount Date and everything else becomes simple.

[Kurt_Bremser]

What @Patrick said: transpose, and it's almost a non-issue. See Maxims 19 & 33.