"In my spare time" vs. "on my spare time"

E.g., "On my spare time I'm working on an essay" or "In my spare time I'm working on an essay".

Which is correct? Both in/on are prepositions, but "in my spare time" sounds more idiomatic to me. A friend corrected me, and now I'm curious.

Which is correct (or are both)?.

I think the correct usage is "in my spare time" (http://idioms.thefreedictionary.com/in+spare+time).

The usage of "on" you're thinking of is probably "on my own time", which has a similar meaning to "in my spare time" (http://idioms.thefreedictionary.com/on+own+time).

Among the "evolved" uses of the preposition "in" includes one that has come to be synonymous with "during," hence the (awful) local news constructs that include sentences like "The victim was shot in the pre-race festivities."