Here is a piece of code which calculates if the flag IsDateDiffLess14_Calculation should be set or not.

SELECT RowID,

CASE WHEN DATEDIFF(DAY, Date2, LEAD(Date1) OVER(PARTITION BY IDNo ORDER BY RowId)) < 14

THEN 1

ELSE 0

END IsDateDiffLess14_Calculation

FROM YourTable

ORDER BY RowID

I am not sure if it's required to have this flag as persistent column in the table. If it is you should modify the script and use it whenever a new row is added to the table to update the previous row only and not the whole table.