output.log

292269008-10-31T10:04:47.327CET
292269008-10-31T10:04:48.332CET
-------------
47499839-07-08T00:00:00.000CET
47499839-07-08T00:00:00.000CET
-------------
2016-10-31T17:17:45.152CET
2016-10-31T17:17:46.153CET
-------------
153548454-10-22T19:38:09.274CEST
153548454-10-22T19:38:10.280CEST
-------------
148430443-05-19T13:11:35.732CEST
148430443-05-19T13:11:36.736CEST
-------------
248679200-02-29T17:17:48.167CET
248679200-02-29T17:17:49.171CET
-------------
2016-10-01T17:17:49.173CEST
2016-10-01T17:17:50.178CEST
-------------
172688176-02-29T17:17:50.179CET
172688176-02-29T17:17:51.181CET
-------------
211481045-09-30T23:59:59.059CEST
211481045-09-30T23:59:59.059CEST
-------------
292269008-10-02T10:04:56.378CET
292269008-10-02T10:04:57.379CET
-------------

Repr.scala

import org.scalatest.FunSuite
import org.scalatest.prop.GeneratorDrivenPropertyChecks
import java.text.SimpleDateFormat
import java.util.{Date, Locale}
import org.scalacheck._

class Repr extends FunSuite with GeneratorDrivenPropertyChecks {
  implicit val cogenDate: Cogen[Date] = Cogen(_.getTime)
  implicit val formatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSz", Locale.ENGLISH)

  test("reproduce") {
    forAll { (i: Int, f: Int => String, g: String => Date) =>
      println(formatter.format(g(f(i))))
      Thread.sleep(1000)
      println(formatter.format(g(f(i))))
      println("-------------")
    }
  }
}

I must be doing something wrong - probably the Cogen instance, but I would have expected g(f(i)) to evaluate to the same value for the same f, g and i, regardless of the time at which the call is made.

Can you additionally print the result of f(i) during your test?