For AppStore apps:

defaults write com.getharvest.harvestxapp TimeFormat hours_minutes
defaults write com.getharvest.harvestxapp TimeFormat decimal
defaults write com.getharvest.harvestxapp TimeFormat server

For apps downloaded directly from GetHarvest:

defaults write ~/Library/Preferences/com.getharvest.harvestx.plist TimeFormat hours_minutes
defaults write ~/Library/Preferences/com.getharvest.harvestx.plist TimeFormat decimal
defaults write ~/Library/Preferences/com.getharvest.harvestx.plist TimeFormat server