DoublingTime.dax

DoublingTime = 

VAR CurrentDay =
    MAX(DailyData[Date] ) 

// Get half of todays value
VAR ThresholdValue =
    CALCULATE(MAX ( DailyData[Confirmed] ), DailyData[Date] = CurrentDay ) / 2

// Get the day when the number of cases was half as much as today
// FILTER returns dates at which the total sum of confirmed cases was below the threshold
VAR DayOfHalf =
    CALCULATE (
        MAX ( DailyData[Date] ),                   
        FILTER (
            ADDCOLUMNS (
                SUMMARIZE ( ALL ( DailyData ), 'Date'[Date] ),
                "Amount", CALCULATE ( SUM ( DailyData[Confirmed] ) )
            ),
            [Amount] <= ThresholdValue
        )
    )

VAR DoublingTime = ( CurrentDay - DayOfHalf ) * 1

// Select only meaningful values
VAR Result =
    IF ( AND ( DoublingTime < 10000, DoublingTime > 0), DoublingTime )
RETURN
    Result

// For details see this blogpost: https://wp.me/p6lgsG-2g8