In my database comment_date and comment_date_gmt are different. comment_date is in UTC+3, but comment_date_gmt in UTC+0.
Why? What do I not right? When I leave the comment I can see "You left a comment 3 hours ago"
The database is correct in this case. comment_date is the datetime of your WordPress timezone setting and comment_date_gmt is the same datetime, but converted to Greenwich Mean Time (GMT). GMT is the timezone all other timezones are offset from while UTC is the time standard. In most practical applications they're interchangeable.
The problem you're probably having is either saving or reading the datetime converted to your local timezone from either GMT or your site's set timezone.
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