I went ahead and tested Google Calendar (finally, it came up) against the free text input I threw at SpongeCell:
- Visiting Granny at 12 on March 1st: passed - SpongCell passed.
- Flying to Cambridge on the 2nd then back on the 4th: failed - SpongCell passed.
- Flying to Boston tomorrow: passed - SpongeCell passed.
- Flying to Boston in a week: failed - SpongeCell failed.
- Flying to Boston on Thursday: passed - SpongeCell passed.
- Flying to Boston a week on Thursday: passed - SpongeCell failed.
- Flying to Boston on the 30th of Feb: same as SpongeCell - placed as 2nd March.
Overall, it seems that SpongeCell is ahead of Google in terms of NL interfaces. It will be interesting to revisit these sites in a while with a new set of tests to see how they have improved.