what is the exact moment of InstllReferrer Library & PackageInfo.firstInstallTime()

Dears, Hi I'm developing advertisement tracking SDK Tools.
I have some question related with Install Stamp.
I'm trying to test & search about that issue, but I can't find any exact answer that I want.
Here is my question. Plz answer theses or tag link related with this qusetion.

1. Google Play Install Referrer Timestamp

   (https://developer.android.com/reference/com/android/installreferrer/api/ReferrerDetails)

• Following the Play Install Referrer Library, getInstallBeginTimestampSeconds() was return "The timestamp in seconds when installation begins.".
  What is the Exact meaning of Installation begins?
  Is it the moment I press the Install button on playstore? Or is it the moment I start downloading from my Android Device?
  Or Is there any Playstore api to record the time when I click the install button on playstore?
• getReferrerClickTimestampSeconds(). Like upper quest I want get exact meaning of "The timestamp in seconds when referrer click happens."
  is it the moment I press the Referrer link on webSite? Or the monent rendering to Playstore?
  Any API that recording the time when Playstore is opened?
• Could there be an average error or exception in getInstallBeginTimestampSeconds () and getReferrerClickTimestampSeconds ()?
  If there is a possibility, I would like you to let me know all the factors that are likely to generate an error. For example, the OS version (which may be ~~~ can happen in SDK 17 or higher, ~~~ can be happen in SDK 23 or higher), Playstore version (in Playstore xx version, it may be ~~~)
  I would like to know if there is any possibility of being investigated or reported.

2. PackageInfo.firstInstallTime

   (https://developer.android.com/reference/android/content/pm/PackageInfo.html#firstInstallTime)

• Following the PacakgeInfo docs, firstInstallTime is "The time at which the app was first installed.".
  As I ask u before, I want exact meaning of that variable. Is it the moment when android system register new package on my own device? Or is it the moment when finish install on my device?