Postgres AT TIME ZONE 'UTC' does NOT do what you think it does

A practical Postgres footgun: AT TIME ZONE 'UTC' converts timestamptz to timestamp without time zone—so month math and equality break unless you apply AT TIME ZONE twice.

And why you may need to repeatAT TIME ZONE 'UTC' twice.

SUMMARY

AT TIME ZONE 'UTC' converts the data type from timestamptz to timestamp (example).
Now you are inadvertently using the timestamp without time zone (aka timestamp) data type, which is markedly discouraged.
There are tons of footguns with timestamp.
For example, the equality of timestamp and timestamptz will always be false.
Adding a month with + INTERVAL '1 months' is timezone-dependent. If your product operates in UTC, you must ensure the timezone is in UTC before adding a month with <timestamptz_column> AT TIME ZONE 'UTC' + INTERVAL '1 months'.... but now the result is a timestamp, not a timestamptz.
To convert the above back to timestamptz in order to avoid issues with equality, you will need to invoke AT TIME ZONE 'UTC again. The final form is: (<timestamptz_column> AT TIME ZONE 'UTC' + INTERVAL '1 months') AT TIME ZONE 'UTC'

Postgres has these 2 timestamp types: timestamp and timestamptz.

timestamp doesn't contain the timezone information. Technically, it doesn't represent a time in the real world. Thinking about it deeply, when we say "5pm" in the real world, it actually means "5pm in our timezone". Even Postgres Wiki says: Don't use timestamp without time zone.

I already know this for quite a while. What I didn't realize is that I might not be able to completely avoid timestamp without time zone.

In many time-series products, you may be building a chart that shows the deltas from month to month and have a SQL like below:

SELECT
  a.month_start,
  (a.value - b.value) AS delta
FROM data a
JOIN data b
ON a.month_start = b.month_start + INTERVAL '1 months'

Now the first issue is: adding months is actually timezone-dependent.

For example, if your timezone is PT, and your product works in UTC, 2026-03-01 00:00:00+00 is equal to 2026-02-28 16:00:00-08. Your machine's default setting is PT. This means Postgres will use 2026-02-28 16:00:00-08, and '2026-02-28 16:00:00-08'::timestamptz + INTERVAL '1 months' will yield 2026-03-28 16:00:00-08. That's not what we want.

Since we operate in UTC, we should convert the timestamp to UTC before adding a month. We modify the SQL to be:

SELECT
  a.month_start,
  (a.value - b.value) AS delta
FROM data a
JOIN data b
ON a.month_start = (b.month_start AT TIME ZONE 'UTC') + INTERVAL '1 months'

It turns out the SQL still doesn't produce the correct result because:

'2026-02-28 16:00:00-08'::timestamptz AT TIME ZONE 'UTC' will convert the data type from timestamptz (with time zone) to timestamp (without time zone) and produces 2026-03-01 00:00:00 (Notice there's no +00 at the end). Example
Comparing a timestamp and timestamptz will always result in false.
1. It makes sense because timestamp is not a real-world point in time. The comparison is technically absurd.
2. It gets even more confusing because, if you perform EXTRACT(EPOCH FROM <timestamp>), they will both produce the same value.

In order to fix it, we have to convert timestamp back to timestamptz with another AT TIME ZONE 'UTC' like below:

ON a.month_start = ((b.month_start AT TIME ZONE 'UTC') + INTERVAL '1 months') AT TIME ZONE 'UTC'

Now the equality will work as you expect it to work.