I have read a few posts specifying that you are allowed to travel for a 90-day period of a 180-day visa.
I have a type D Schengen visa for 365 days. I have been in Italy for 5 months but have not travelled to another Schengen territory yet.
Would I still be able to travel to other countries or has my window of opportunity disappeared?
You can go to other Schengen countries; you haven't used up any of the time you have available to spend in those countries.
Anyone who holds a D visa of a Schengen country may travel to other Schengen countries for up to 90 days in any 180-day period. Note the word other. Time spent in the country issuing the D visa does not count against the time allowed in other countries.
This was enacted by Regulation 265/2010, which amends the Schengen convention as follows (Article 1, paragraph 2):
(2) Article 21 is amended as follows:
(a) paragraph 1 is replaced by the following:
‘1. Aliens who hold valid residence permits issued by one of the Member States may, on the basis of that permit and a valid travel document, move freely for up to three months in any six-month period within the territories of the other Member States, provided that they fulfil the entry conditions referred to in Article 5(1)(a), (c) and (e) of Regulation (EC) No 562/2006 of the European Parliament and of the Council of 15 March 2006 establishing a Community Code on the rules governing the movement of persons across borders (Schengen Borders Code) ( 20 ) and are not on the national list of alerts of the Member State concerned.
(b) the following paragraph is inserted after paragraph 2:
‘2a. The right of free movement laid down in paragraph 1 shall also apply to aliens who hold a valid long-stay visa issued by one of the Member States as provided for in Article 18.’
This last paragraph is important: your D visa is the long-stay visa mentioned there, and it is this paragraph that gives the D visa the same status as a residence permit for the purpose of allowing travel to other Schengen countries.
