BMIs and start time of 0.0

[dorchard]

I've been looking into the expected properties of models implementing the BMI interface and looking at what is described in the bmi-tester. I noticed that the tester expects the start time to be 0.0 (https://github.com/csdms/bmi-tester/blob/c4f5c994a13f6fd010ef5ba25fdcaffa893222be/src/bmi_tester/_tests/stage_1/time_test.py#L16-L21) I wondered if that's universal and if so, why is a start time needed? I can imagine that maybe some models might want to start somewhere in real history, perhaps at the beginning of a sensor record or at the start of the industrial revolution or 1970 or something like this. In which case the bmi-tester would not like it! I was wondering if anyone had any more info, thoughts, or knowledge they could share with me (a newbie) (maybe @mdpiper or @mcflugen?)

[hrajagers]

Hi Dominic,

Well spotted! The start time doesn't have to 0.0, but the BMI documentation for get_start_time states: "The start time in BMI is typically defined to be 0.0." In many of our own real world models, we use a reference date which is often equal to the simulation start time and hence the start time itself will often turn out to be equal to 0.0, but that will not always be the case. Even if the model returns 0.0 (or any other value) you will need the result of get_time_units to put that value in proper (historical) context with a reference date. There is more that one could say about time, such as which calendar is being used ... this is especially relevant for historical simulations (before October 1582; gregorian/julian), highly schematized cases (e.g. 360 days/year) or simulations of other planetary bodies (what's a year or even a day?).

[dorchard]

Thanks for this Bert. That makes a lot of sense. Now I wonder about this test in the bmi-tester then: do we really want to be enforcing 0.0 (even if that is the most common value)?

[mcflugen]

@dorchard To solve the problem of a reference date we had discussed a couple of options. One was to add a new BM function (I liked get_start_date because it sounded like "get star date" like Kirk might say). The other was to include the start date in the unit string obtained from get_time_units (e.g. "milliseconds since 1970-01-01T00:00:00").

I don't think we ever had a vote but the consensus seemed to be to go with the second option.

Either way, I think the bmi-tester should be updated to allow for non-zero start times.