The author (may Allah have mercy on him) said: Chapter on dhikr in the morning and the evening.
What he meant by morning and evening was at the beginning of the day, and at the end of the day and the beginning of the night. The morning begins when dawn breaks and ends when the sun has risen at the forenoon. And the evening begins with ‘Asr prayer and ends with ‘Isha’ prayer or thereabouts.
End quote from Sharh Riyadh as-Saaliheen
Ibn al-Qayyim (may Allah have mercy on him) said:
Chapter on remembering Allah (dhikr) at the two ends of the day (as mentioned in Hud 11:114), which are the times between Fajr and sunrise, and between ‘Asr and Maghrib. Allah, may He be glorified and exalted) says:
“O you who believe! Remember Allah with much remembrance. And glorify His Praises morning and afternoon” [al-Ahzaab 33:41-42]. The “afternoon”: al-Jawhari said: This is the time after ‘Asr until Maghrib. And Allah, may He be exalted, says (interpretation of the meaning): “and glorify the praises of your Lord in the ‘Ashiy (i.e. the time period after the midnoon till sunset) and in the Ibkaar (i.e. the time period from early morning or sunrise till before midnoon)” [Ghaafir 40:55].
The ibkaar is the beginning of the day and the ‘ashiy is the end of the day. And Allah, may He be exalted, says (interpretation of the meaning): “and glorify the praises of your Lord before the rising of the sun, and before its setting” [Ta-Ha 20:130]. This interpretation is what is mentioned in the hadeeths about saying such and such in the morning and evening. What is meant is before the sun rises and before it sets. Therefore the time for these adhkaar is after Fajr and after ‘Asr.