adding / subtracting time in JS

Lately I was playing with JS and I found something interesting. That is what I wrote into chrome console:

today = new Date()
-> Mon Apr 29 2013 13:06:01 GMT+0200 (CEST)
DAY = 1000 * 3600 * 24
-> 86400000
today - 2 * DAY
-> 1367060761452
today + 2 * DAY
-> "Mon Apr 29 2013 13:06:01 GMT+0200 (CEST)172800000"

And I am wondering why am I getting different types of answer depending on the type of operation – adding / subtracting. When I do something like that:

today - (-2) * DAY

everything is fine. Is there any ideology, or is it a bug?

today + 2 * DAY
uses concatenation of strings. If you want do it properly, use today.getTime().

Example:

tomorrow = new Date()
tomorrow.setTime(today.getTime() + DAY)