[R] Challenge extracting months

[Kwesi Quagraine]

Hello, I have a data with two variables nodes and index, I want to extract
3 months seasons, with a shift of 1 month, that is, DJF, JFM, FMA etc to
OND. Was wondering how to go about it. Kindly find attached the data as csv.
Any help will be appreciated.

Regards,
​Kwesi​

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