perf(rule_engine): improve the time offset parser

[lafirest]

Fixes EMQX-9969

Summary

🤖 Generated by Copilot at c8c2793

Refactor emqx_calendar.erl to enhance readability and style.

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
• 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
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• 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 5364499479

• 8 of 10 (80.0%) changed or added relevant lines in 1 file are covered.
• 28 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.004%) to 81.683%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx/src/emqx_calendar.erl	8	10	80.0%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_dashboard/src/emqx_dashboard_desc_cache.erl	1	94.44%
apps/emqx_dashboard/src/emqx_dashboard_monitor.erl	1	66.21%
apps/emqx_gateway/src/emqx_gateway_schema.erl	1	94.44%
apps/emqx/src/emqx_broker.erl	1	88.2%
apps/emqx/src/emqx_cm.erl	1	92.05%
apps/emqx/src/emqx_metrics.erl	1	88.21%
apps/emqx/src/emqx_ocsp_cache.erl	1	82.53%
apps/emqx_oracle/src/emqx_oracle.erl	4	80.0%
apps/emqx/src/emqx_reason_codes.erl	17	85.29%

Totals
Change from base Build 5356448716:	-0.004%
Covered Lines:	30217
Relevant Lines:	36993

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💛 - Coveralls