Question: Is standing [during] the last third [of the night in prayer] best? [Please advise] with clarity of the time when the last third of the night is.
Response: The Prophet (sal-Allaahu ‘alayhi wa sallam) said:
And the last third of the night differs with the changing of the seasons – Winter, Spring, Summer and Autumn – so it is not possible to pinpoint a specific time [when that would always be]. The rule in this [case] would be to add up the hours from sunset [at Maghrib] until the arrival of Fajr [the following day], then divide that equally into three, and with that the last third of every night will be made apparent to you.
 For example, if the time for Maghrib is 7pm, and Fajr is 4am the following day, the number of hours between the two would be 9 hours. So dividing 9 into 3 would give us 3. With that, the last third of the night will be determined as having started 3 hours before Fajr, at 1am.