datetime - Writing to firestore in firebase cloud-function shifts my dates

Question

Working on flutter/dart project. Calling cloud function from client side and passing 2 dates which are converted toISO8601String.

Printing dates to console while executing cloud-function to double check. They are always valid. After creating document in firestore, the dates are shifted by 1 hour.

I guess because of my current timezone offset which is UTC+1. In summer I had same issue where firestore was shifting my dates by 2 hours because of summer time, resulting in offset UTC+2.

My cloud-functions are deployed at region: europe-west3, which according to firebase docs is Frankfurt. Same time zone as mine (Central European Standard Time). But when I create:

const now = new Date();

Result is one hour less than my local time. Why is that when we are in the same time zone with same offset?

Reading documents from firestore on client side results in inaccurate dates, which is pretty bad for me since I need it to be accurate.

I was searching everywhere but didn't find anything that really helped me. I was trying to construct the date object with many ways but the result was always the same, can somebody please explain me why and help me out? Thanks.

question from:https://stackoverflow.com/questions/65540933/writing-to-firestore-in-firebase-cloud-function-shifts-my-dates

Answer 1

The Java Object Date does not actually contain timezone information, it's just a number of milliseconds since the "epoch" (01.01.1970 00:00:00 UTC) and thus an absolute point in time.

It's converted into a valid time every time it's output to a user.

So it seems that the reason for the time shift is that your PC or the browser you are viewing your firestore documents from is set to the wrong timezone.

For further information, take a look at this question or the java documentation.