Whereas English has only one word for time, other European languages more carefully distinguish between time as instance ("How many times?") and time as sequence ("How much time?").
The ancient Greeks, who thought a great deal about time (among many subjects), also had two different words that we English-speakers are forced to translate with the single word "time." Since there is a very profound difference between those two words, and because time is an important theological subject in the New Testament (itself written in Greek), it is useful to examine this difference.
The first Greek term is chronos, meaning time on the move, time as before and after, time as the future passing through the present and so becoming the past. From this Greek word chronos we derive such English terms as chronic, chronicle, and chronology. Thus, we call an illness chronic if it lasts a long time. A chronicle is an account of events through a sequence of time. Chronology is the itemized, studied measurement of time.
Indeed, measurement is one of the distinguishing characteristics of chronos, which is a quantitative concept. Time that cannot be measured is not chronos. Time in this sense is dimensional, quantifiable; it can be "accounted for."
In addition to chronos, however, the Greeks also spoke of time as a moment, time as occasion, time as qualitative rather than quantitative, time as significant rather than dimensional. This second word for time is kairos.
Strictly speaking, we don't measure kairos. We don't ask someone, for example, "How much Christmas did you have?" We inquire, rather, "what sort of Christmas did you have?" With kairos we employ the category of qualis, not quantum.
The reason that time in the sense of kairos cannot be measured is because it is always a now. A now is obviously indivisible; an instant is, so to speak, too brief to account for. By the time you stop to measure a now, it is already gone. Now is punctuated by a swift, indecipherable passage from this to that. Furthermore, if now cannot be measured, it can also not be counted. It is futile, for example, and probably a threat to sanity, to ask how many nows there are in an hour. Unlike the past and the future, nonetheless, the now really exists. Indeed, now is the only time that does exist. In the strictest sense, "there's no time like now."
Kairos, because it is present, is an icon of eternal life. To experience the now, after all, one must be alive. The dead know nothing of now. Therefore, the now, the kairos, is an icon of the life of heaven. Indeed, eternal life is an everlasting now, in which there is no sequence, no before and after.
Eternity is not a long time. Strictly speaking, there is simply no length to it. Nothing elapses. The infinite is not measurable. Thus, "when we've been there ten thousand years/ bright shining as the sun/ we've no less days to sing God's praise/ than when we've first begun."
(The Rev. Fr. Patrick Henry Reardon, 2009; article found at http://www.orthodoxytoday.org/articles5/ReardonChronos.php,)