Great job buddy !! the idea you had looks awesome :) What i would suggest is to implement aswell a function that also analyses text even if it is on another line...I believe a lot will do this mistake so why not implement it ! :)

quoting your code:

function findMentionLine(comment) {
    return comment.split(/\r?\n/).find(line => line.includes('@' + REMAIND_ME_USERNAME)) || null;
}

function getFutureDateByTextAndContext(text, context) {
    return chrono.parseDate(text, context);
}

function getFurureDateByCommentTextAndContextDate(commentText, contextDate) {
    const mentionText = findMentionLine(commentText);
    const future = getFutureDateByTextAndContext(mentionText || '', moment(contextDate));
    return future ? moment(future) : null;
}

In terms of coding that will not be even a big issue for you. Just read each line of the comment and assign it in to an array of strings (each line of the comment as one element of the array) then loop through the array as per array.length (that will let the code know that if the array.length > 1 there are more lines to look at) and do a text matching into a couple IF statement containing some keyword as (Day, Hour, Minute, etc...) to know where to start looking for the time of the set reminder then pass the result to the other functions.

Again great job man !! i will for sure use it !! and by the way i already forked your repo to keep up to date with whatever you will implement :)