added ISO 8601 clarications, corrected spelling

Author: gbinal

schema.md

@@ -283,8 +283,10 @@ Further Metadata Field Guidance (alphabetical by JSON field)
 **Cardinality** | (1,1)
 **Required** | Yes, always
 **Accepted Values** | ISO 8601 Date
-**Usage Notes** | Dates should be [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601) of least resolution. In other words, as much of YYYY-MM-DDThh:mm:ss.sTZD as is relevant to this dataset. If this file is brand-new, enter the **issued** date here as well.
-**Example** |  `{"modified":"2012-01-15"}`
+**Usage Notes** | Dates should be [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601) of least resolution. In other words, as much of YYYY-MM-DDThh:mm:ss.sTZD as is relevant to this dataset. If this file is brand-new, enter the **issued** date here as well.  
+  
+If there is a need to reflect that the dataset is continually updated, ISO 8601 formatting can account for this by [giving the duration](http://en.wikipedia.org/wiki/ISO_8601#Durations).  For instance, `P1D` for daily, `P2W` for every two weeks, and `PT5M` for every five minutes.  
+**Example** |  `{"modified":"2012-01-15"}` or `{"modified":"P1D"}`
 
 {: .table .table-striped}
 **Field** | **PrimaryITInvestmentUII**
@@ -337,8 +339,12 @@ Further Metadata Field Guidance (alphabetical by JSON field)
 **Cardinality** | (0,1)
 **Required** | Yes, if applicable
 **Accepted Values** | ISO 8601 Date
-**Usage Notes** | This field should contain an interval of time defined by start and end dates.  Dates should be formatted as pairs of {start datetime/end datetime} in the [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601) format. ISO 8601 specifies that datetimes can be formatted in a number of ways, including a simple four-digit year (eg. 2013) to a much more specific YYYY-MM-DDTHH:MM:SSZ, where the T specifies a seperator between the date and time and time is expressed in 24 hour notation in the UTC (Zulu) time zone. (e.g., 2011-02-14T12:00:00Z/2013-07-04T19:34:00Z). Use a solidus ("/") to separate start and end times.
-**Example** |  `{"temporal":"2000-01-15T00:45:00Z/2010-01-15T00:06:00Z"}`
+**Usage Notes** | This field should contain an interval of time defined by start and end dates.  Dates should be formatted as pairs of {start datetime/end datetime} in the [ISO 8601](http://en.wikipedia.org/wiki/ISO_8601) format. ISO 8601 specifies that datetimes can be formatted in a number of ways, including a simple four-digit year (eg. 2013) to a much more specific YYYY-MM-DDTHH:MM:SSZ, where the T specifies a seperator between the date and time and time is expressed in 24 hour notation in the UTC (Zulu) time zone. (e.g., 2011-02-14T12:00:00Z/2013-07-04T19:34:00Z). Use a solidus ("/") to separate start and end times.  
+  
+If there is a need to reflect that the dataset is continually updated, ISO 8601 formatting can account for this by [with repeating intervals](http://en.wikipedia.org/wiki/ISO_8601#Time_intervals).  For instance, updated monthly starting in January 2010 and continuing through the present would be represented as: `R/2010-01/P1M`.
+
+Updated every 5 minutes beginning on February 15, 2010 would be represented as: `R/2010-02-15/PT5M`.  
+**Example** |  `{"temporal":"2000-01-15T00:45:00Z/2010-01-15T00:06:00Z"}`  or `{"temporal":"R/2000-01-15T00:45:00Z/P1W"}`  
 
 {: .table .table-striped}
 **Field** | **theme**
@@ -347,7 +353,7 @@ Further Metadata Field Guidance (alphabetical by JSON field)
 **Required** | No
 **Accepted Values** | Array of strings
 **Usage Notes** | Separate multiple categories with a comma. Could include [ISO Topic Categories](http://www.isotopicmaps.org/).  
-**Example** |  `{"theme":["vegetables"]}` or if muplitle categories, `{"theme":["vegetables","produce"]}`
+**Examples** |  `{"theme":["vegetables"]}` or if multiple categories, `{"theme":["vegetables","produce"]}`
 
 {: .table .table-striped}
 **Field**       | **title**