Set datetime timezone when posting data via JSON api
In DateTimeNormalizer, the inferface format is RFC3339, which is Y-m-d\TH:i:sP as an input format string.  It also accepts the ISO8601 PHP string, which is Y-m-d\TH:i:sO

Try using strings like in your request '2020-02-20T13:15:00+01:00'.

What I suspect is happening, is that since you aren't specifying the UTC offset, it is being interpreted as '+00:00'

The denormalizer also allepts a format string with the request to parse the value.  I believe it is called format

See also DateTimeNormalizerTest and EntityTestDatetimeTest for some more examples.