If I could simplify them enough to read well, I might use a calendar with a backward or forward arrow on it, or an hourglass with same.

When I need ideas, I like to search existing icon repositories.

Here are some ideas:

- https://www.iconfinder.com/search/?q=past

- https://www.iconfinder.com/search/?q=future

I would use visual metaphors from a music player. Back (<<) Pause/Present(||) Forward (>>)

Thought about it myself but I am afraid it is too widely used for navigation in pagination. So IMHO it might be misleading. Thanks for the suggest, though.

Past: A sundial

Now: a digital watch/alarm clock/iPad with time showing

Future: A robot (maybe with an iPad for a head)/terminator coming out of a portal.

Would these icons represent data filters or actions?

Filters for actions: past = done, now = in focus, future = deal with later.

checkmark on top of thin square, square, dotted outline of a square?

Good idea, but added an arrow (?) to the future. Made a sketch, what do you think? http://cl.ly/image/2m121A351M0F