ThoughtSpot_Administration_Guide_4.2 lists greatest and least under mixed functions (can be used with text and numeric data types) on page 299. However looking through available formulas in UI, greatest/least only appears under numeric.
Steph Fisher , yes the issue here is related to the chasm trap. Because of the chasm trap, we could have multiple values of date1 for each date2 and multiple values of date2 for each date1 and hence cannot determine this. You can check out the if formula over 2 fields from the same table.