JSFiddle - React, Tailwind, and code Playground

JavaScript

var baseDate = new Date();
    timeElapsed = "01:23:00", // time elapsed
    timeElapsedParts = timeElapsed.split(':'),
    timeElapsedDate = new Date(),
    timeElapsedInMilliseconds = 0,
    timeTotal = "07:34:00", // total time
    timeTotalParts = timeTotal.split(':'),
    timeTotalDate = new Date(),
    timeTotalInMilliseconds = 0,
    timeDifferenceInPercentage = 0;

baseDate.setHours(0);
baseDate.setMinutes(0);
baseDate.setSeconds(0);
baseDate.setMilliseconds(0);

timeElapsedDate.setHours(timeElapsedParts[0]);
timeElapsedDate.setMinutes(timeElapsedParts[1]);
timeElapsedDate.setSeconds(timeElapsedParts[2]);
timeElapsedDate.setMilliseconds(0);

timeTotalDate.setHours(timeTotalParts[0]);
timeTotalDate.setMinutes(timeTotalParts[1]);
timeTotalDate.setSeconds(timeTotalParts[2]);
timeTotalDate.setMilliseconds(0);

timeElapsedInMilliseconds = (timeElapsedDate.getTime() - baseDate.getTime());
timeTotalInMilliseconds = (timeTotalDate.getTime() - baseDate.getTime());

timeDifferenceInPercentage = (Math.round(((timeTotalInMilliseconds - timeElapsedInMilliseconds) / timeTotalInMilliseconds) * 10000) / 100);

console.log(timeDifferenceInPercentage + '%');