I was recently asked about the use of this field, in a user export csv. The value is in epoch time format. Running it through a converter, such as this one:
https://www.epochconverter.com/, reveals the date and time in user readable format. For example, after changing a user's password the filed contained 1505944117. Putting this value in the converter gave a date and time of:
GMT: Wednesday, September 20, 2017 9:48:37 PMYour time zone: Wednesday, September 20, 2017 5:48:37 PM GMT-04:00 DST