This appears to be some kind of weird mix of a text and a numeric representation that includes milliseconds since 1-Jan-1970 UTC. My suggestion is to extract the main numeric part of that text (700000 in the example you point to) and use the datetime constructor and 'ConvertFrom'.
The time zone portion appears to be pretty much useless on its own. There are any number of time zones that at some point in the year have a UTC offset of 5 hours (+0500 from the example) and no way to disambiguate that. The description you point to says, "the number of milliseconds in the GMT time zone", which is already almost certainly wrong, it's UTC not GMT, but in any case, I'd just ignore any time zone portion and use some other information to correctly localize the time stamp.