What time format is this & how to convert to readable format?

This is one of those questions that I'm sure will have a quick answer but I can't seem to find it.

Using the Facebook Graph API, the opening hours for a business are returned in JSON using this format:

"hours": {
  "mon_1_open": 406800,
  "mon_1_close": 437400,
  "tue_1_open": 493200,
  "tue_1_close": 523800,
  "wed_1_open": 579600,
  "wed_1_close": 610200,
  "thu_1_open": 61200,
  "thu_1_close": 91800,
  "fri_1_open": 147600,
  "fri_1_close": 178200,
  "sat_1_open": 237600,
  "sat_1_close": 264600,
  "sun_1_open": 324000,
  "sun_1_close": 345600
}

The hours should be 0900 - 1730 weekdays, 1000-1730 Saturdays & 1000-1600 Sundays.

I can se开发者_如何转开发e there are 30600 seconds between opening and closing on weekdays, so it's a number of seconds since something, but I'm not sure since what.

What kind of time format is this, and how can I convert it into something usable in PHP?

Bonus: Why would they use a timestamp "since" something, seeing as the hours recur weekly?

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I'm not sure what TimeZone you're in, but looking at it, hey all seem to be exact second counts originating from 00:00:00Z Thursday morning. I'd say that puts you at about the PST timezone.