Earth Engine filtering - creating monthly composites, but returning UNIX timestamp rather than YMD

Question

I'm new to Earth Engine and struggling with a problem related to ee.Date formats. I am filtering image collections to pass into later analyses, including regression. However, when I later try to add a time band, to permit that regression, I receive errors. We've tracked this down to the way we have defined system:time_start being wrong in the monthly filtering, because our code pulls the system:time_start in date format but really it should be the time stamp in milliseconds since the UNIX epoch.

Here is the code we are using to create our monthly composites:

var interval = 1;
var month_list = ee.List.sequence(1,12, interval);
var year_list = ee.List.sequence(2000, 2014); 

var monthlyCHIRPS =  ee.ImageCollection.fromImages(
  year_list.map(function (ynz) {
  return month_list.map(function(mnz){
  var w = CHIRPSCollection.filter(ee.Filter.calendarRange(ynz, ynz, 'year'))
                        .filter(ee.Filter.calendarRange(mnz, ee.Number(mnz).add(interval), 'month'))
                        .sum();
  return w.set('year', ynz)
                        .set('month', mnz)
                        .set('date', ee.Date.fromYMD(ynz,mnz,1))
                        .set('system:time_start',ee.Date.fromYMD(ynz,mnz,1))
                        .set('ID',ee.Date.fromYMD(ynz,mnz,1))

})
}).flatten())

print(monthlyCHIRPS, 'Monthly CHIRPS');

I believe the error is in setting the system:time_start, in that currently we pull the date from YMD, in the same format. What I want to do, is somehow pull the date, but convert it back into a UNIX timestamp as in the original images, so I can later add a timeband.

Does anyone know how I can correct this code to do that, or have an alternative code for creating monthly composites that would permit this?

Answer 1

Use .millis() to convert a ee.Date to a long number in milliseconds

.set('system:time_start',ee.Date.fromYMD(ynz,mnz,1).millis())

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