at what time

when did you last see him?

since when have you been interested?

at or on which (referring to a time or circumstance)

Saturday is the day when I get my hair done

at or during the time that

I loved maths when I was at school

after which; and just then (implying suddenness)

he had just drifted off to sleep when the phone rang

in view of the fact that; considering that

why bother to paint it when you can photograph it with the same effect?

although; whereas

I'm saying it now when I should have told you long ago