To read your settings, run:
defaults read ~/Library/Containers/com.getharvest.harvestxapp/Data/Library/Preferences/group.com.getharvest.Harvest.Documents.plist
To change the time format to HH:MM, run
defaults write ~/Library/Containers/com.getharvest.harvestxapp/Data/Library/Preferences/group.com.getharvest.Harvest.Documents.plist TimeFormat hours_minutes
Restart the Harvest App, and it should now display HH:MM (I tested this)
To change back to decimal, run:
defaults write ~/Library/Containers/com.getharvest.harvestxapp/Data/Library/Preferences/group.com.getharvest.Harvest.Documents.plist TimeFormat decimal
Thanks to Ben for this tip!