Question

I’m using .NET to generate a JSON file that has many Dates in it.

For compression, I want to store them as milliseconds since Jan 1, 1970 and not strings, then convert them to Javascript Dates. But .Net’s idea of milliseconds since 1970-01-01 don’t match Javascript:

Javascript:

Date.parse("2012-05-15T13:57:57.0000000+00:00")
1337090277000

VB.Net:

Date.Parse("2012-05-15T13:57:57.0000000+00:00").Subtract(New Date(1970,1,1)).TotalMilliseconds
1337101077000.0

The difference is 10800 seconds. The difference at 1970-01-01 is 0 and changes over time.

Is there a way to compute Javascript’s idea of milliseconds-since-epoch from within .Net?

Answer 1

You are comparing apples to oranges.

This is exactly what you would get in javascript as well when in UTC+3 (Israel):

Date.parse("2012-05-15T13:57:57.0000000+00:00") - new Date(1970,1,1)
//1334419077000

This is because when you do new Date in javascript, that’s according to the machine’s timezone. It looks like it’s the same for vb.net.

---

You would get the correct number in javascript with:

Date.parse("2012-05-15T13:57:57.0000000+00:00") - Date.UTC(1970,1,1)
//1334411877000

In VB.net

    Dim a As Date
    a = Date.Parse("2012-05-15T13:57:57.0000000+00:00").ToUniversalTime()
    Dim b As Date
    b = New Date(1970, 2, 1, 0, 0, 0, DateTimeKind.Utc)
    a.Subtract(b).TotalMilliseconds
    '1334411877000 Same as javascript
    'Note that in javascript, month as 1 is same as 2 in VB.net because months start at 0 in javascript