Convert a time offset string -HHMM into offset minutes.

convertOffset.js

var convertOffset = function( offset ){
    if( typeof offset !== 'string' ) return;
    var hours = parseInt( offset.substr( -4, 2 ), 10 );
    var minutes = parseInt( offset.substr( -2, 2 ), 10 );
    var negative = ( offset.substr( -5, 1 ) === '-' );
    var offset_in_minutes = hours * 60 + minutes;
    if( negative ) offset_in_minutes = offset_in_minutes * -1;
    return offset_in_minutes;
};