In case anyone else runs into the same issue, here is the workaround I've come up with. Like Nick mentioned, my original timestamp didn't store any time zone information, so I had to use the more general TO_TIMESTAMP() function, perform the calculation in UTC, and then convert back to Pacific.

SELECT
    TO_TIMESTAMP('2025-01-30 23:19:45.000') as ts
    ,CONVERT_TIMEZONE('UTC', 'America/Los_Angeles', DATEADD(DAY, 90, CONVERT_TIMEZONE('America/Los_Angeles', 'UTC', ts))) as ts_pdt