I'm trying to call a Webservice with a DateTime format like this : "\/Date(x)\/" where 'x' is the millisecond value since 1970.

How to generate a DateTime in this format ?

Here is what i've done :

    dateValidation_long = dateValidation_string.getTime();
    String a = '\\/Date(';
    String b = ')\\/';
    modifierEntiteDeFacturation3C_request.dateValidation = a + dateValidation_long + b;

Here is the result in my console : 

    \/Date(1505904139000)\/

Then, i'm trying to JSON.serialize this date, to send it via a Webservice, and this is the result : 

    dateValidation":"\\/Date(1505904534000)\\/"

If someone got a solution, a tips, tell me :)

Have a good day