What is format of timestamp in Chrome performance log message?

I am using Selenium with ChromeDriver to get the performance log from Chrome.

caps = DesiredCapabilities.CHROME
caps['loggingPrefs'] = {'performance': 'ALL'}

driver = webdriver.Chrome(desired_capabilities=caps)
driver.get("some.website")
time.sleep(5)

for entry in driver.get_log('performance'):
    print(entry)

Inspecting these messages, I found a timestamp field with weird value (779922.902049). So my question is what is the format of this timestamp field?

{'message': '{"message":{"method":"Network.responseReceived","params":{"frameId":"29983.1","loaderId":"29983.1","requestId":"29983.1","response":{"connectionId":0,"connectionReused":false,"encodedDataLength":0,"fromDiskCache":fals
e,"fromServiceWorker":false,"headers":{"Access-Control-Allow-Origin":"*","Content-Type":"text/plain;charset=US-ASCII"},"mimeType":"text/plain","protocol":"data","securityState":"unknown","status":200,"statusText":"OK","url":"data:
,"},"timestamp":779922.902049,"type":"Document"}},"webview":"e6d532fe-f007-4397-bf28-4c4a26c79e4d"}', 'level': 'INFO', 'timestamp': 1523607170083}

{'message': '{"message":{"method":"Network.loadingFinished","params":{"encodedDataLength":0,"requestId":"29983.1","timestamp":779922.902067}},"webview":"e6d532fe-f007-4397-bf28-4c4a26c79e4d"}', 'level': 'INFO', 'timestamp': 152360
7170083}

Note that there is another timestamp field in these above message which is milliseconds since epoch. But it is not the field I am asking.