fix(calendar): make date parse error reason more sense

[lafirest]

Fixes EMQX-10592

Summary

🤖 Generated by Copilot at 8b0bbbd

Refactored and documented a date conversion function in emqx_calendar.erl.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update

[coveralls]

Pull Request Test Coverage Report for Build 5793585199

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 33 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.03%) to 81.426%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_bridge_gcp_pubsub/src/emqx_bridge_gcp_pubsub_client.erl	1	94.74%
apps/emqx_ft/src/emqx_ft_assembler.erl	1	87.23%
apps/emqx_ft/src/emqx_ft_assembly.erl	1	97.12%
apps/emqx_ft/src/emqx_ft_storage_exporter_fs.erl	1	80.43%
apps/emqx_gateway_mqttsn/src/emqx_mqttsn_channel.erl	1	73.78%
apps/emqx_management/src/emqx_mgmt_util.erl	1	13.79%
apps/emqx/src/emqx_broker.erl	1	88.33%
apps/emqx/src/emqx_connection.erl	1	83.47%
apps/emqx/src/emqx_limiter/src/emqx_limiter_server.erl	1	89.36%
apps/emqx/src/emqx_quic_stream.erl	1	75.0%

Totals
Change from base Build 5788466989:	-0.03%
Covered Lines:	31977
Relevant Lines:	39271

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💛 - Coveralls