Add time range to checker.

docs/csaf_checker.md

@@ -16,10 +16,11 @@ Application Options:
   -v, --verbose                  Verbose output
   -r, --rate=                    The average upper limit of https operations per second (defaults to unlimited)
   -y, --years=YEARS              Number of years to look back from now
+  -t, --timerange=RANGE          RANGE of time from which advisories to download
   -H, --header=                  One or more extra HTTP header fields
       --validator=URL            URL to validate documents remotely
       --validatorcache=FILE      FILE to cache remote validations
-      --validatorpreset=         One or more presets to validate remotely (default: mandatory)
+      --validatorpreset=         One or more presets to validate remotely (default: [mandatory])
   -c, --config=TOML-FILE         Path to config TOML file
 
 Help Options:
@@ -48,6 +49,7 @@ insecure         = false
 verbose          = false
 # rate           # not set by default
 # years	         # not set by default
+# timerange      # not set by default
 # header         # not set by default
 # validator      # not set by default
 # validatorcache # not set by default
@@ -64,8 +66,37 @@ type 1: warning
 type 2: error
 ```
 
-The checker result is a success if no checks resulted in type 2, and a failure otherwise. 
+The checker result is a success if no checks resulted in type 2, and a failure otherwise.
 
+`years` and `timerange` allows only checking advisories from a given time interval.
+It is only allowed to specify one off them. 
+`years`  looks number of years back from now. `timerange` values allow finer controls:
+
+1. Relative. If the given string follows the rules of being a [Go duration](https://pkg.go.dev/time@go1.20.6#ParseDuration)
+    the time interval from now minus that duration till now is used. 
+    E.g. `"3h"` means checking the advisories that have changed in the last three hours.
+
+2. Absolute. If the given string is an RFC 3339 date timestamp the time interval between
+   this date and now is used. 
+   E.g. `"2006-01-02"` means that all files between 2006 January 2nd and now going to be
+   checked. 
+   Accepted patterns are:
+   - `"2006-01-02T15:04:05Z"`
+   - `"2006-01-02T15:04:05+07:00"`
+   - `"2006-01-02T15:04:05-07:00"`
+   - `"2006-01-02T15:04:05"`
+   - `"2006-01-02T15:04"`
+   - `"2006-01-02T15"`
+   - `"2006-01-02"`
+   - `"2006-01"`
+   - `"2006"`
+
+   Missing parts are set to the smallest value possible in that field.
+
+3. Range. Same as 2 but separated by a `,` to span an interval. e.g `2019,2024`
+   spans an interval from 1st January 2019 to the 1st January of 2024.
+
+All interval boundaries are inclusive.
 
 ### Remarks