WordPress goes to great lengths in order to provide us with a localized environment, displaying dates and times in the correct format and even language. However, since it uses its own timezone setting, some native PHP functions are no longer applicable (or don’t return correct results), such as the date() function which returns a date/time string depending on the timezone configuration of PHP itself, ignoring the WordPress setting. Recently, I retrieved from a third-party API a timestamp, which I needed to convert to a WordPress-correct timestamp. WordPress already provides a great function, date_i18n() (i18n stands for internationalization, as there are 18 letters between the beginning i and the ending n), which seemingly supports GMT/UTC timestamps, but once I tried it I quickly realized its $gmt parameter only applies if you don’t provide your own timestamp.
There is however, a little known function called get_gmt_from_date() which almost does what I needed, but not quite. The problem is that it won’t accept a timestamp, but a string in the format Y-m-d H:i:s (e.g. 2018-03-19 23:59:59). No problem! Let’s create our own little function that will wrap get_gmt_from_date()
Source: https://managewp.org/articles/17283/how-to-convert-a-gmt-utc-timestamp-to-local-time-in-wordpress
Willie Chiu's Blog 