[tz] [PROPOSED] Improve coverage of version 4 interoperability
eggert at cs.ucla.edu
Wed Mar 17 17:21:36 UTC 2021
* tzfile.5: Mention problems with version 3-only readers and
version 4 files, and update other interoperability coverage.
tzfile.5 | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/tzfile.5 b/tzfile.5
index 8ee6cfb..b4952d5 100644
@@ -243,10 +243,13 @@ Readers that only understand Version 1 must ignore
any data that extends beyond the calculated end of the version
1 data block.
-Writers should generate a version 3 file if
+Other than version 1, writers should generate
+the lowest version number needed by a file's data.
+For example, a writer should generate a version 3 file
+if the file does not contain a truncated leap second table
+and so does not use version 4 features, but
TZ string extensions are necessary to accurately
-model transition times.
-Otherwise, version 2 files should be generated.
+model transition times so the file does need version 3 features.
The sequence of time changes defined by the version 1
header and data block should be a contiguous subsequence
@@ -269,7 +272,7 @@ and
This is for compatibility with POSIX requirements for
time zone abbreviations.
-When reading a version 2 or 3 file, readers
+When reading a version 2+ file, readers
should ignore the version 1 header and data block except for
the purpose of skipping over them.
@@ -328,6 +331,10 @@ for the next time zone east, e.g.,
for permanent Atlantic Standard Time (\-04).
+Some readers designed for earlier versions reject version 4 files,
+because they require complete leap second tables that do not record
Some readers ignore the footer, and instead predict future
timestamps from the time type of the last transition.
As a partial workaround, a writer can output more transitions
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