Moment & Timezone convert local time fails

Question

This is in an Ionic based hybrid app on NodeJS.

Trying to convert a local time as specified by a user input to another timezone, yet it fails:

static MTL_local_time_to_server(aDateTime:moment.Moment):moment.Moment{
    console.log(aDateTime.format('MMMM Do YYYY, h:mm:ss a'));

    const localTime:moment.Moment = momenttz.tz(aDateTime, momenttz.tz.guess());
    console.log(localTime.format('MMMM Do YYYY, h:mm:ss a'), momenttz.tz.guess());
    const returnTime:moment.Moment = momenttz(localTime).tz("Europe/Berlin");
    console.log(returnTime.format('MMMM Do YYYY, h:mm:ss a'));
    return returnTime;
}

Prints

April 22nd 2019, 12:00:00 am 
April 22nd 2019, 12:00:00 am America/Los_Angeles 
April 22nd 2019, 9:00:00 am

Answer 1

A few things:

• You've typed the output of moment.tz as a string , but it's actually a Moment object.

• When you call JSON.stringify on a Moment object, it returns the output of .toISOString() , which is always in UTC. (The Z indicates UTC in the ISO 8601 format.)

• It's not clear if your input is a string or a Date object. If it's a string , then the Z indicates UTC, so it will always be interpreted as UTC. If it's a Date object, then the point in time represented by that Date object will be used - which depends on how you constructed the Date object. Either way, the value you're showing is UTC based, not local time.

• It's not clear exactly what you're trying to accomplish. From your variable names, it would seem like you are trying to convert localDateTime to localTime , which logically would give the same value if they were both "local".

  • If you're trying to convert a value from local time to Berlin time, then:

     moment(yourInput).tz('Europe/Berlin') 

  • If you're trying to convert a value from Los Angeles time to Berlin time, then:

     moment.tz(yourInput, 'America/Los_Angeles').tz('Europe/Berlin') 

  • If you're trying to convert a value from UTC to local time, you don't need moment-timezone at all:

     moment.utc(yourInput).local() 

• If you need string outputs, then you should call the format function to produce a string. What you showed here looks like you are logging Moment objects not strings.