RTC does not save 'tm_isdst'

[ilikecake]

CircuitPython version

Adafruit CircuitPython 8.2.8 on 2023-11-16; Adafruit Feather ESP32S3 4MB Flash 2MB PSRAM with ESP32S3

Code/REPL

import time
import rtc

now = time.struct_time((2024, 1, 12, 21, 51, 40, 4, 12, 0))
print(now)
rtc.RTC().datetime = now
now = time.localtime()
print(now)

Behavior

struct_time(tm_year=2024, tm_mon=1, tm_mday=12, tm_hour=21, tm_min=51, tm_sec=40, tm_wday=4, tm_yday=12, tm_isdst=0)
struct_time(tm_year=2024, tm_mon=1, tm_mday=12, tm_hour=21, tm_min=51, tm_sec=40, tm_wday=4, tm_yday=12, tm_isdst=-1)

Description

It appears that the tm_isdst is not saved as part of the RTC. It does not thrown an error, but it always returns -1 when getting the time from the RTC. From the example above, I set the tm_isdst to 0, but when I get the time from the RTC, it returns -1.

I am not sure if this is something specific to the ESP32, or with the RTC library more generally.

Additional information

No response

[dhalbert]

When you set the RTC, the number of seconds since some start date (the "epoch") is recorded. All the other info is discarded. One of tm_isdst's main uses is to disambiguate times that fall near the boundaries of the date-time when DST starts or stops. See https://stackoverflow.com/a/47333019/142996.

Since the timezone and location info are not saved in the RTC, the DST state is not really interesting. You could save the timezone and location info if needed. Note that the timezone does not include all the info needed to determine DST state either. For instance, some states in the US observe daylight savings time, and others in the same timezone do not (e.g., Arizona).

[dhalbert]

Closing for now.

[dhalbert]

Reopening: this question came up again and the reason the tm_isdst value is discarded should really be documented.