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kotlinlang

I want to submit a PR to allow `AbstractLongTimeSource` (<https://github.com/JetBrains/kotlin/blob/c56893d9c2c9be51df78acf98e6b6984b10407c5/libraries/stdlib/src/kotlin/time/TimeSources.kt#L28>) &amp; `AbstractDoubleTimeSource`  be able to provide `TimeMark` s for _reading_s done outside of the time source.

This is useful if you already have some readings you want to convert into time marks to be able to easily do duration calculations or interface with some other system that did those readings already.

For example
```object UptimeMillisSource : AbstractLongTimeSource {
  override fun read() = SystemClock.uptimeMillis()
}```
If I already have readings of `SystemClock.uptimeMillis()`  from another system, I should be able to call:
```UptimeMillisSource.[ timeMarkOf | readingOf | readingToTimeMark | convert ](some: Long): TimeMark```
Any thoughts or I go ahead &amp; submit a PR?

In general, you need a YouTrack issue first

Yeah I thought so. I brought this here cos lates contribution guidelines says to start a discussion here. <https://youtrack.jetbrains.com/issue/KT-76762>