This assumes that your month values in A1 to L1 are text values and not dates formatted as mmm.
Sheet1
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P |
1 | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | | Dec | 59 | < Current Month manually input into cell N1 |
2 | 8 | 3 | 5 | 1 | 10 | 1 | 5 | 9 | 4 | 4 | 2 | 7 | | | 59 | < Current Month automatically the month of today |
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Spreadsheet Formulas |
Cell | Formula | O1 | =SUM(A2:INDEX(A2:L2,MATCH(N1,A1:L1,0))) | O2 | =SUM(A2:INDEX(A2:L2,MATCH(TEXT(TODAY(),"mmm"),A1:L1,0))) |
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