Whenever the minimum time is set the time specification is assumed to coincide with the one set in the control (i.e. ignored), which is what is described for QDateTimeEdit::setDateTime. On the contrary when setting the max time, local time is assumed in all cases.

The relevant code is as follows for setMinimumTime:

void QDateTimeEdit::setMinimumTime(const QTime &min)
{
    Q_D(QDateTimeEdit);
    if (min.isValid()) {
        const QDateTime m(d->minimum.toDate(), min, d->spec);
        setMinimumDateTime(m);
    }
}

while for setMaximumTime:

void QDateTimeEdit::setMaximumTime(const QTime &max)
{
    Q_D(QDateTimeEdit);
    if (max.isValid()) {
        const QDateTime m(d->maximum.toDate(), max);
        setMaximumDateTime(m);
    }
}

Incidentally this makes QDateTimeEdit and QTimeEdit pretty finicky and quite frankly next to unusable in most real-world scenarios.