docs: explain how ecalc handles time

docs/docs/about/modelling/setup/index.md

@@ -7,7 +7,15 @@ description: Guide on how to setup an eCalc™ model
 # Set up an eCalc Model
 This section describes how to create your own eCalc™ model file. 
 
-There are six separate sections which make up each model, these being: 
+First some general information about [File formats and syntax](/about/modelling/setup/file_format_and_syntax/index.md),
+with some specific details about writing [Expressions](/about/modelling/setup/file_format_and_syntax/expressions.md)
+in the YAML file.
+
+Then some information about how [eCalc™ interprets the time input](/about/modelling/setup/the_concept_of_time_in_ecalc.md)
+given in various parts of the YAML file.
+
+In the end there are some descriptions of the different sections that the YAML file consists of. There are six separate
+sections: 
 
 | Input                                                              | Function                                                                                                                                                                                          |
 |--------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|

docs/docs/about/modelling/setup/the_concept_of_time_in_ecalc.md

@@ -0,0 +1,64 @@
+---
+title: How eCalc™ handles time
+sidebar_position: 2
+description: Time in eCalc™ - guide and description
+---
+
+# Time periods in eCalc™
+eCalc™ does calculations between a global start date and a global end date. This global time period will be split into
+many smaller time periods, where each of them follow immediately after each other, based on other dates encountered in
+the YAML file and in the various time series resource files. 
+
+
+## Finding the global start and end dates
+
+The global start and end dates can be specified directly in the YAML file. 
+eCalc™ will first check if the global start and end dates are specified by the users. This is done using the
+[START](/about/references/START.md) and [END](/about/references/END.md) keywords in the YAML file. If the 
+[START](/about/references/START.md) and [END](/about/references/END.md) keywords are not specified, the dates will be 
+determined by the time series data given in [TIME_SERIES](/about/references/TIME_SERIES.md).The earliest date and latest date encountered within the 
+time series data with [INFLUENCE_TIME_VECTOR](/about/references/INFLUENCE_TIME_VECTOR.md) set to True will be considered the global start date and global end 
+date, respectively. 
+
+:::note
+Any dates defined in the YAML file (e.g. in [ENERGY_USAGE_MODEL](/about/references/ENERGY_USAGE_MODEL.md)) will not
+influence the global start and end dates.
+:::
+
+:::note
+To have complete control over the period of time for which eCalc™ will do calculations, it is recommended to specify the
+global start and end dates yourself using the [START](/about/references/START.md) and [END](/about/references/END.md) keywords
+in the YAML file.
+:::
+
+## Splitting the global time period into smaller time periods
+The period of time between the global start and end dates, will be split into many smaller time periods. This is done
+by finding:
+* all other dates in all the time series data with
+  [INFLUENCE_TIME_VECTOR](/about/references/INFLUENCE_TIME_VECTOR.md) set to True.
+* all dates defining temporal models in the YAML file (e.g. in 
+  [ENERGY_USAGE_MODEL](/about/references/ENERGY_USAGE_MODEL.md), [FUEL](/about/references/FUEL.md) or
+  [VARIABLES](/about/modelling/setup/variables.md)).
+
+When all **N** dates are found and sorted, (**N-1**) time periods will be generated. The first time period will start
+at the global start date and end at the next date in the sorted list. The second time period will start where the
+previous ends (start is inclusive, end is exclusive), and so on until the last time period which will end at the global
+end date.
+
+The time series input given in the [TIME_SERIES](/about/references/TIME_SERIES.md) may not have values for all the
+generated periods. Any missing values will be found using the chosen
+[INTERPOLATION_TYPE](/docs/about/references/INTERPOLATION_TYPE). Be aware that if a time series has
+[INFLUENCE_TIME_VECTOR](/about/references/INFLUENCE_TIME_VECTOR.md) set to False, there is a possibility that dates can
+be dropped here.
+
+:::note
+The last row given for the variables in a single csv file is considered to be the values for the variables in the period
+starting at that date. If no later dates are given in any other csv files, or no [END](/about/references/END.md) date is
+given, the last row in that file is in reality irrelevant - it will not be used for
+anything. But, if an end date is defined through the [END](/about/references/END.md) keyword, that last row will be the values for the 
+variables in that additional added period of time.
+:::
+
+:::note
+If one csv file ends several periods of time before another csv file, the last row in the first csv file will be the
+:::