When Is Nafl Prayer Not Allowed?
Question

I read in one of the questions on your site about the times when prayer is forbidden. Could you please explain what these times are by the clock, so as to put my mind at rest?
Praise be to Allah.Determining the forbidden time to offer nafl prayer by clock
Determining the  times when voluntary prayer is forbidden varies from one country to another, and from one season to another. Hence we cannot explain what these times are by the clock for all lands and in all seasons. 
When is nafl prayer not allowed?
But we will explain the general principles which will make it easy for every Muslim to work out when these times are. Hence we say that the times when prayer is forbidden are three: 

From when dawn breaks until approximately a quarter of an hour after the sun has risen . You can find out the time of sunrise from the timetables that are available in all countries. 
Approximately a quarter of an hour before the time for Zuhr prayer begins, until the time for Zuhr begins. 
After you have prayed `Asr – even if it is an hour after the time for it began – until the disk of the sun has set completely. So the beginning of the time when prayer is forbidden is when one has prayed `Asr, not the beginning of the time for `Asr prayer, because the Muslim may offer `Asr prayer some time after the time for it begins. In that case the Muslim may offer voluntary prayers so long as he has not yet prayed `Asr, even if the time for `Asr has begun. Ibn Qudamah (may Allah have mercy on him) said in al-Mughni (1/429): “We do not know of any difference of opinion concerning that among those who say that prayer is not allowed after `Asr.”

Hadiths about the forbidden times to offer nafl prayer
The evidence for these times is mentioned in several hadiths, among the clearest and most comprehensive of which is the lengthy hadith which was narrated by Imam Muslim in his Sahih (832) from `Amr ibn `Abasah (may Allah be pleased with him), that the Prophet (peace and blessings of Allah be upon him) said to him: 
“Pray Fajr, then refrain from praying until the sun has risen and becomes high, for when it rises, it rises between the horns of the Shaytan and at that time the disbelievers prostrate to it. Then pray, for the prayer is witnessed and attended until the shadow of a spear falls directly north (i.e., noon). Then refrain from praying, for at that time Hell is stoked up. Then when the shadow moves forward, pray, for the prayer is witnessed and attended, until you have prayed `Asr. Then refrain from praying until the sun has set, for it sets between the horns of the Shaytan and at that time the disbelievers prostrate to it.” 
Can you offer prayers for which there is a reason at the forbidden times?
We should point out that what is forbidden is prayer that is purely voluntary at these times. As for prayers for which there is a reason , such as “greeting the mosque” (tahiyyat al-masjid ) or the two rak`ahs after wudu or the two rak`ahs after tawaf and so on, they may be offered at any time according to the more correct of the two scholarly opinions.”  
And Allah knows best.