Hi Manish,
Given that developing task list on SharePoint Online is related to development, Microsoft does provide chargeable custom solution and development support. However, I would provide some helpful links for your reference. Perhaps some of the other users on this community will have experience useful to your issue. To solve the issue efficiently, you may also want to post your question to the MSDN forums for a better response for custom solutions.
References:
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How can I calculate the number of days between two dates? For example, DueDate vs Today.
Download: SharePoint Online for Office 365 Developer Guide
Thanks,
Ida Qiu