when I'm declaring 'time' as variable of integer type it is turning orange.Is it a predefined function?I'm not using RTC library also.What could be the reason for the variable 'time' turning orange?
It's a keyword in one of the libraries? Just ignore it. I always ignore the colouring.
I can reproduce what you found, and I haven't found the word "time" in any of my libraries. Oh well.
Does the same for me: It must be listed in one of the keywords.txt files in the Arduino install or your custome libraries, but I don't have the energy to look right now. But those are just text files for the colour coding in the IDE and afaik, don't do anything more than that. So it's not that "time" is reserved or anything.
That said, maybe you should in any case use a more meaningful name for your variable, to better describe what time you are referring to, eg timeSinceHeaterOn or whatever :)