parse javascript date to elixir format

Question

I have some saved dates in JavaScript using new Date() that looks like:

"Sun Feb 24 2019 14:44:20 GMT+0200 (Eastern European Standard Time)"

I'm trying to parse these to Elixir DateTime ; I didn't find anything in "timex" that can help and I already know that I can use DateTime.from_iso8601 but for dates saved using new Date().toISOString() but what i need is to parse the above string.

Thanks in advance

Answer 1

You can use elixir binary pattern matching to extract the date parts and parse using Timex's RFC1123 format. The RFC1123 is the format eg Tue, 05 Mar 2013 23:25:19 +0200 . Run h Timex.Format.DateTime.Formatters.Default in iex to see other formats.

iex> date_string = "Sun Feb 24 2019 14:44:20 GMT+0200 (Eastern European Standard Time)"

iex> <<day_name::binary-3,_,month_name::binary-3,_,day::binary-2,_,year::binary-4,_,time::binary-8,_::binary-4,offset::binary-5,_,rest::binary>> = date_string

iex> Timex.parse("#{day_name}, #{day} #{month_name} #{year} #{time} #{offset}", "{RFC1123}")

iex> {:ok, #DateTime<2019-02-24 14:44:20+02:00 +02 Etc/GMT-2>}

Pattern matching: The binary-size are in byte sizes. 1 byte == 1 character. For instance to get 3-character day_name the size is 3. Underscores (_) is used to pattern match the spaces in the date format

Updated answer to use binary-size rather than bitstring-size for simplicity

Answer 2

I didn't find anything in "timex" that can help

The Timex Parsing docs say that you can use strftime sequences, eg %H:%M:%S , for parsing. Here's a list of strftime characters and what they match.

Here's a format string that I think should work on javascript Dates:

  def parse_js_date() do
    Timex.parse!("Sun Feb 24 2019 14:44:20 GMT+0200 (Eastern European Standard Time)", 
                 "%a %b %d %Y %H:%M:%S GMT%z (%Z)",
                 :strftime)
  end

Unfortunately, %Z doesn't want to match the time zone name, which causes Timex.parse!() to spit out an error. It looks like %Z in Elixir only matches one word, eg a timezone abbreviation EET . Therefore, my simple, clean solution is spoiled.

What you can do is chop off the time zone name before parsing the date string:

def parse_js_date_string() do

    [date_str|_tz_name] = String.split(
        "Sun Feb 24 2019 14:44:20 GMT+0200 (Eastern European Standard Time)",
        " (", 
        parts: 2
    )

    Timex.parse!(date_str,
                 "%a %b %d %Y %H:%M:%S GMT%z",
                :strftime)
  end

In iex:

~/elixir_programs/my$ iex -S mix
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Compiling 1 file (.ex)
Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> My.parse_js_date_string()
#DateTime<2019-02-24 14:44:20+02:00 +02 Etc/GMT-2>

iex(2)>