All timestamps (both those sent in requests and those returned in responses) are formatted using RFC 3339 timestamps. The preferred way to pass in a date is by converting the time to UTC such as this: 2018-01-01T08:00:00Z.
In cases where timestamps are rounded to a given day, you may omit the time component. So instead of 2018-01-01T00:00:00Z you can use 2018-01-01. Any date specified with no time is assumed to be 00:00:00 UTC.
Our APIs support the subset of dates after the start of the Unix epoch: 1970-01-01T00:00:00+00:00 (00:00:00 UTC on January 1, 1970).