Bitcoin Pro API - How do you format start/end time for Historic Data? I don't understand how to format the start and end time per https://docs.pro.coinbase.com/#get-historic-rates

The sample time is "1415398768" which makes no intuitive sense to me, and when I tried looking up ISO 8601 time formats, none of the formats I found have 10 digits like the bitcoin sample.
This time format is known as Unix time. It is the number of seconds since 00:00 UTC, January 1st, 1970.