How to calculate date difference from two columns but with different rows and a condition?

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Based on the example of dataframe below, I would like to calculate difference between two datetime for certain index and its cumulative. The expected results are as in the column diff_days and cum_diff days

index date_a date_b diff_days cum_diff_days
1 1/1/2023 NaT NaT -
1 NaT NaT NaT -
1 NaT 3/1/2023 2 2
2 4/1/2023 NaT NaT -
2 NaT NaT NaT -
2 NaT 6/1/2023 2 4
3 7/1/2023 NaT NaT -
3 NaT 8/1/2023 1 5
3 9/1/2023 NaT NaT -
3 NaT NaT NaT -
3 NaT 11/1/2023 2 7

I have checked the other post where it calculates the difference between two dates, unfortunately that one is when the date is in the same row. For my case, I wanted to understand how to calculate the dates if it's on different rows at different column since just subtracting it with df['diff_days'] = df['date_a'] - df['date_b'] will produce aNaTresults. I really appreciate if there is someone enlighten me on this problem.



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