Negative `engagementDuration` Values Due to Time Synchronization Issues

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We use Cloudflare Zaraz to integrate GA4 for event monitoring. While analyzing the underlying GA4 event data, we discovered an interesting phenomenon: a significant number of `engagement_time_msec` values are appearing as negative.


## Root Cause Analysis

After investigating, we identified that this occurs when there's a time synchronization discrepancy between the user's device and Cloudflare Worker (Zaraz's execution environment). Specifically, negative `engagement_time_msec` values appear when the user's device time is ahead of the Worker time.

We conducted a test to reproduce this behavior:

1. Set client time 4 minutes ahead of actual time
2. Focus on a page, where client time is used to [mark engagementStart](https://github.com/managed-components/google-analytics-4/blob/main/src/index.ts#L97)


3. When the user leaves the page, a `visibilitychange` event triggers, and `engagementDuration` is [calculated on the worker side](https://github.com/managed-components/google-analytics-4/blob/main/src/index.ts#L130) as approximately `now - engagementStart`. In cases where the client time is ahead of the server time, this results in a negative `engagementDuration`
4. This can be verified by querying the underlying event data in BigQuery for GA4, where `engagement_time_msec` reflects this time difference


It's worth noting that the issue also affects accuracy when client time lags behind server time, resulting in inflated engagement duration values.

## Suggested Solution

It might be worth considering using timestamps consistently (either all client-side or all edge-side) when calculating `engagementDuration`. This could help address potential issues since client-side time discrepancies are fairly common.

Start in src/index.ts at the engagementStart logic around line 97 and the worker-side engagementDuration calculation around line 130. Reproduce the discrepancy with a client clock four minutes ahead, then compare the resulting GA4 engagement_time_msec values in BigQuery. Done means the chosen timestamp strategy no longer produces negative or inflated engagement durations under clock differences.

Written by the indexing model from the issue text.