group_deltatime vs. group_elapsed

[Karen K]

Can someone say what the difference between group_deltatime and group_elapsed is? In which case should I use one or the other for newly defined observation types?

[Vince Skahan]

From the upgrade guide:

• A "delta time" is the difference between two times, for example the amount of uptime (difference between start and current time), or the amount of daylight (difference between sunrise and sunset).
  

From poking around a git clone:

• deltatime seems to have been first defined in 2.7.0 (Oct-2014) and used then in station.py and later in almanac.py
  
• elapsed seems to have been first defined in 2.1.0 (Jan-2013) but I can't find it ever being used or documented

It might be a backward compatibility thing that never got cleaned up.   Perhaps the name 'elapsed' was changed to 'deltatime' to be more accurate and the old name never got deleted (????)