Use express, moment, moment-timezone

  
const moment = require('moment-timezone');  
  
  
app.get('/moment', (req, res) => {  
  const datestr = '2020-07-01';  
  
  res.status(200).json({  
    local_offset: moment(datestr).utc(),  
    local_unix: moment(datestr).unix(),  
    zone0_unix: moment(datestr).zone(0).unix(),  
    zone8_unix: moment(datestr).zone(8).unix(),  
    timezone0_unix: moment.tz(datestr, 'GMT').unix(),  
    timezone8_unix: moment.tz(datestr, 'Asia/Taipei').unix(),  
  });  
});  
  
// No use, just for keep   
function dateForTimezone(offset, d) {  
  // Copy date if supplied or use current  
  d = d? new Date(+d) : new Date();  
  
  // Use supplied offset or system  
  offset = offset || -d.getTimezoneOffset();  
  // Prepare offset values  
  var offSign = offset < 0? '-' : '+';   
  offset = Math.abs(offset);  
  var offHours = ('0' + (offset/60 | 0)).slice(-2);  
  var offMins  = ('0' + (offset % 60)).slice(-2);  
  
  // Apply offset to d  
  d.setUTCMinutes(d.getUTCMinutes() - offset);  
  
  return offSign + offHours + ':' + offMins;  
  
  // Return formatted string  
  return d.getUTCFullYear() +   
    '-' + ('0' + (d.getUTCMonth()+1)).slice(-2) +   
    '-' + ('0' + d.getUTCDate()).slice(-2) +   
    'T' + ('0' + d.getUTCHours()).slice(-2) +   
    ':' + ('0' + d.getUTCMinutes()).slice(-2) +   
    ':' + ('0' + d.getUTCSeconds()).slice(-2) +   
    '.' + ('000' + d.getUTCMilliseconds()).slice(-3) +  
    offSign + offHours + ':' + offMins;   
    
}

IMPORT

Server timezone 0

Client time zone +8

Server run result:

  
{  
  "local_offset": "2020-06-30T16:00:00.000Z",  
  "local_unix": 1593532800,  
  "zone0_unix": 1593532800,  
  "zone8_unix": 1593532800,  
  "timezone0_unix": 1593561600,  
  "timezone8_unix": 1593532800  
}

Client run result:

  
{  
  "local_offset": "2020-07-01T00:00:00.000Z",  
  "local_unix": 1593561600,  
  "zone0_unix": 1593561600,  
  "zone8_unix": 1593561600,  
  "timezone0_unix": 1593561600,  
  "timezone8_unix": 1593532800  
}

Conclusion

  
moment().zone().unix()  Auto fix zone / utc  
  
moment("").tz("GMT").unix()  Auto fix zone / utc  


  
moment().tz("", "GMT").unix()  No auto fix zone / utc