The timestamp should be UTC with a timezone.
So, in your first example of 'start="20190408170000 +0300"' then the time should be 5pm +3 hours TZ. Most programmes use GMT since it's easy to convert to localtime/TZ time.
As an example, a Polish sample file I have says 'start="20190514202500 +0200".
If you have perl you can play with the values and conversions:
perl -MDateTime -MDateTime::Format::Strptime -de 42
$strp = DateTime::Format::Strptime->new(pattern=>'%Y%m%d%H%M%S %z")
$x = $strp->parse_datetime("20190523180000 +0100")
print $x->offset
print scalar localtime $x->epoch
print scalar gmtime $x->epoch
^D
