convert local time to utc and add to utc datetime moment - react-native

Question

I have a user input where the user enters year, month and day and I create a date object
let userDate = new Date(year, month, day)
and then from a user input where the user enters minutes and hours (into variables minutes and hours)
I need to convert this date and time into UTC timestamp, using moment and I'm not sure about it. Can I just do
let utcTimestamp = moment.utc(date.toISOString()).hour(hours).minutes(minutes);
For example: if a user enters a date of 13-Mar-2018 and time of 8:45 AM (in GMT timezone), then I could use the above line of code to get UTC timestamp as I can directly add hours and minutes to the date
if a user enters a date of 13-Aug-2018 and time 8:45 (which is GMT +1, due to daylight savings time change) then with above line I might be creating a wrong date.

Answer 1

... I create a date object
let userDate = new Date(year, month, day)
Be careful here, you need to subtract 1 from the month, as they are numbered 0-11 by the Date instead of the usual 1-12.
and then from a user input where the user enters minutes and hours (into variables minutes and hours)
I need to convert this date and time into UTC timestamp, using moment ...
You don't need Moment for this. Just call .toISOString() on the Date object. It implicitly converts to UTC before generating the string.
var utcString = new Date(year, month-1, day, hours, minutes).toISOString();
... Can I just do
let utcTimestamp = moment.utc(date.toISOString()).hour(hours).minutes(minutes);
No, that doesn't make sense. That would only work if the date was entered in local time but the time was entered in UTC. Such a difference in behavior would surely be confusing to your user - especially if the UTC date was different than the local date.
... if a user enters a date of 13-Aug-2018 and time 8:45 (which is GMT +1, due to daylight savings time change) then with above line I might be creating a wrong date.
The conversion from local time to UTC will automatically take into account any daylight saving adjustment that is being observed by the local time zone. You do not need to do any additional adjustment on your own.

Answer 2

This is a snippet i used to convert an outlook calendar event from UTC to local time. The same Technique could be used for other scenarios.
//Get the users timezone
let timeZone = item.originalStartTimeZone;
//Get the start datetime in UTC time
let timeStart = item.start.dateTime;
//Calculate the negative offset.
let offset = - timeStart.localeCompare(timeZone);
//Add the calculated offset to the UTC start time
let localStartTime = addHours(timeStart, offset);