What
is the timing of the evening adhkaar? What is the best time for reciting
them? Can they be made up later on if one forgets them?

Praise be to Allaah. 

The
evening (al-masaa’) is a
lengthy period, from after ‘asr prayer until ‘ishaa’ prayer – all
of this period is called masaa’
(evening). It is the same whether one recites the dhikr at the beginning
of this time or at the end, apart from those adhkaar which the reports
specifically say should be recited at night, such as reciting Aayat
al-Kursi at night. Adhkaar which should be recited specifically at night
should be recited at night, and those which should be recited specifically
during the day should be recited during the day. With regard to making
them up if one forgets them, I hope that he will be rewarded for doing
that.