Date / Time difference expressions and calculation
I want to have an easy method to calculate number of days, hours, etc. relative to a given date.
(For an easy to use powerfull home automation system this should be a kernel function. Workarounds with complicated scripts are not satisfied.
See also https://forum.pimatic.org/topic/3916/date-6-months.)
Maybe this requirement could be satisfied by extending the dateTimeDevice by a referenceDate attribute.
If the referenceDate is set, a difference variable to the actual date(time) is provided.
For example:
referenceDate = 2018-01-04
all other settings default
results in $datetime.difference = -97
Maybe there could also be an option for defining the output format of the difference variable. [days|hours|minutes|seconds]
@saxnpaule Sorry but a fixed reference date doesn’t realy help. What needed is a variable value which can be changed by rule or variable input device.
I don’t really understand. Please describe your usecase more in detail.
Scenario:
You create a “TÜV” datetime device with the reference date 2018-07-01
Now you could define a rule like
when trigger:$datetime.difference < 1 then ...
@saxnpaule The reference value must be changeable automatically from external actions. Assume that the reference value comes from a sensor or is requested by a shell script. With plain variables this is possible but as far as I know not with device attributes, these are readonly. Last but not least the user wants a easy and natural UI like described in my inital post. So at the moment I can not agree to close this thread.
I still see no concrete scenario. Could you please decribe a real usecase?
@saxnpaule The user wants to get notified x days after the battery of a sensor was changed or he had pressed a button to turn on an actuator. This should also works if the pimatic system is down for some time in between. Further the user wants to see how many days or hours those devices are in operations since these events.
The user wants to get notified x days after the battery of a sensor was changed
The change off a battery is a manual step. Afterwards set the date in the device. Thats it.
or he had pressed a button to turn on an actuator
You could define a rule that writes the current date/time into a variable that is displayed in the frontend
@saxnpaule You misunderstand me completely 😞
Then start a chat and try to explain in german please 
The change off a battery is a manual step. Afterwards set the date in the device. Thats it.
In my case a variable $changedate is set automatically.
You could define a rule that writes the current date/time into a variable that is displayed in the frontend
The problem is how to get the elapsed time from setting this variable to now?
How could pimatic know when the battery is changed? Could you provide a sample rule? Would be interesting for others too I think.
Getting the elapsed time? Just display the calculated difference of the datetime device by a variableDevice.
What about providing a function to the pimatic core that allows to calculate the difference between two dates.
Thats quite easy. Just tested it:
when ... then set $duration = diffDate("2018-01-01", "2018-02-01", "days")
This also works
$last-change = "2018-02-15 17:53:40"
when every 60 minutes then set $duration = diffDate($last-change, date(), "days")
when trigger:$duration < 1 then ...
For a quick solution just add the function to the VariableManager in the variables.coffee located in pimatic\lib folder. For a permanent solution ask @mwittig to add this to the next pimatic release.
diffDate:
args:
dateIn1:
type: "string"
optional: no
dateIn2:
type: "string"
optional: no
format:
type: "string"
optional: yes
exec: (dateIn1, dateIn2, format) ->
date1 = Date.parse(dateIn1)
date2 = Date.parse(dateIn2)
diff = date1 - date2
if format
switch format
when "seconds"
diff = diff / 1000
when "minutes"
diff = diff / 1000 / 60
when "hours"
diff = diff / 1000 / 60 / 60
when "days"
diff = diff / 1000 / 60 / 60 / 24
return Math.ceil(diff)
Regarding the use of Math.round() or Math.ceil() there needs to be a further discussion. I would suggest to use ceil() for positive differences and round() for negative ones.
That means the difference in days will be at least 1 as long as it isn’t <=0.
Half a day after passing the scheduled date, the difference will be -1 days.
By using always the round() function, the difference will be 0, 12 hours before and after the configured “schedule”.
@saxnpaule I am not a pimatic develloper but that looks promising.
@mwittig Can you confirm this?
@heizelmann said in Date / Time difference expressions and calculation:
@mwittig Can you confirm this?
Yes, I had a similar approach in my mind as I followed the conversation in the past days, but had no time to write a detailed answer as I am currently travelling. Actually, the solution suggested by @SaxnPaule is smart and good to go in my opinion! @SaxnPaule if you file a PR this will be great!
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
@saxnpaule and @mwittig
Thanks a lot 
I am looking forward to the update and I will test it as soon as it is available.
With the current implementation the call diffDate("2018-01-01", "2018-01-02", "days") yields -1. I find this a bit counterintuitive as, typically, I’d regard the two dates as a start and end date of an interval, i.e. 1.
What do you think?
EDIT: or make the difference an absolute value … In this case the result for the example is 1 no matter which date is passed in first.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law
Mhh I think it depends on the usecase.
If you use the difference for dates in past and future to have a countdown or something like that, positive and negative values make totally sense. How many days until the defined date is reached or how many days have already passed.
Also the initial usecase. How many days are gone scince date x and now.
By the order of the attributes you are able to decide whether you want to have positive or negative values.
