Ordinarily, if a date/time string is syntactically valid but contains out-of-range field values, an error will be thrown. For example, input specifying the 31st of February will be rejected.
During a daylight-savings-time transition, it is possible for a seemingly valid timestamp string to represent a nonexistent or ambiguous timestamp. Such cases are not rejected; the ambiguity is resolved by determining which UTC offset to apply. For example, supposing that the TimeZone parameter is set to America/New_York, consider
=> SELECT '2018-03-11 02:30'::timestamptz; timestamptz ------------------------ 2018-03-11 03:30:00-04 (1 row)
Because that day was a spring-forward transition date in that time zone, there was no civil time instant 2:30AM; clocks jumped forward from 2AM EST to 3AM EDT. PostgreSQL interprets the given time as if it were standard time (UTC-5), which then renders as 3:30AM EDT (UTC-4).
Conversely, consider the behavior during a fall-back transition:
=> SELECT '2018-11-04 02:30'::timestamptz; timestamptz ------------------------ 2018-11-04 02:30:00-05 (1 row)
On that date, there were two possible interpretations of 2:30AM; there was 2:30AM EDT, and then an hour later after the reversion to standard time, there was 2:30AM EST. Again, PostgreSQL interprets the given time as if it were standard time (UTC-5). We can force the matter by specifying daylight-savings time:
=> SELECT '2018-11-04 02:30 EDT'::timestamptz; timestamptz ------------------------ 2018-11-04 01:30:00-05 (1 row)
This timestamp could validly be rendered as either 2:30 UTC-4 or 1:30 UTC-5; the timestamp output code chooses the latter.
The precise rule that is applied in such cases is that an invalid timestamp that appears to fall within a jump-forward daylight savings transition is assigned the UTC offset that prevailed in the time zone just before the transition, while an ambiguous timestamp that could fall on either side of a jump-back transition is assigned the UTC offset that prevailed just after the transition. In most time zones this is equivalent to saying that "the standard-time interpretation is preferred when in doubt".
In all cases, the UTC offset associated with a timestamp can be specified explicitly, using either a numeric UTC offset or a time zone abbreviation that corresponds to a fixed UTC offset. The rule just given applies only when it is necessary to infer a UTC offset for a time zone in which the offset varies.