A timestamp like 2019-07-18 01:21:29.826484 will be truncated to 2019-07-18. But for your Rails application 2019-07-18 01:21:29.826484 is 2019-07-17 22:21:29.826484 at your time zone (GMT -03:00). So it should be truncated to 2019-07-17 instead.So, you should convert the timestamp to your current Rails time zone before extracting the date.

In everyday usage, MST is often referred to as Mountain Time (MT) or the Mountain Time Zone. This can add a bit of confusion as the term Mountain Time does not differentiate between standard time and Daylight Saving Time, so Mountain Time switches between MST and MDT in areas that use DST during part of the year.

Some areas in British Coloumbia, including Creston, Fort Nelson, Fort St. John, and Dawson Creek use Mountain Standard Time all year.

Most North American time zones also have generic terms, including Pacific Time (PT), Central Time (CT), Eastern Time (ET), and Atlantic Time (AT).

Intl.DateTimeFormat().resolvedOptions().timeZone

For example, you might want to use the browser’s knowledge of the user’s current time zone to group a collection of elements by date.