diff --git a/docs/features/index.md b/docs/features/index.md
index 331436ea..8e7e0479 100644
--- a/docs/features/index.md
+++ b/docs/features/index.md
@@ -355,6 +355,7 @@
 - [PPL Rename Command](sql/ppl-rename-command.md)
 - [PPL Rex and Regex Commands](sql/ppl-rex-and-regex-commands.md)
 - [PPL Spath Command](sql/ppl-spath-command.md)
+- [PPL Timechart Command](sql/ppl-timechart-command.md)
 - [Security Lake Data Source](sql/security-lake-data-source.md)
 - [SQL Error Handling](sql/sql-error-handling.md)
 - [SQL Pagination](sql/sql-pagination.md)
diff --git a/docs/features/sql/ppl-timechart-command.md b/docs/features/sql/ppl-timechart-command.md
new file mode 100644
index 00000000..217ec345
--- /dev/null
+++ b/docs/features/sql/ppl-timechart-command.md
@@ -0,0 +1,174 @@
+# PPL Timechart Command
+
+## Summary
+
+The PPL `timechart` command creates time-series visualizations by aggregating data into time buckets. It supports various span intervals, rate-based aggregation functions (`per_second`, `per_minute`, `per_hour`, `per_day`), custom timestamp fields, and grouping by categorical fields. The command is essential for performance monitoring, log analysis, and trend visualization in OpenSearch.
+
+## Details
+
+### Architecture
+
+```mermaid
+graph TB
+    subgraph "PPL Query Processing"
+        A[PPL Query with timechart] --> B[Parser]
+        B --> C[Timechart AST Node]
+        C --> D[Query Rewriter]
+        D --> E{per_* function?}
+        E -->|Yes| F[Transform to sum + eval]
+        E -->|No| G[Standard aggregation]
+        F --> H[Chart Command Handler]
+        G --> H
+        H --> I[OpenSearch Aggregation Query]
+    end
+    
+    subgraph "Span Processing"
+        J[Span Definition] --> K{Span Type}
+        K -->|Fixed| L[Static calculation]
+        K -->|Variable| M[Dynamic timestampdiff]
+        L --> N[Time Bucket Creation]
+        M --> N
+    end
+```
+
+### Data Flow
+
+```mermaid
+flowchart LR
+    A[Source Data] --> B[Time Bucketing]
+    B --> C[Aggregation]
+    C --> D{Has per_* function?}
+    D -->|Yes| E[Rate Normalization]
+    D -->|No| F[Direct Output]
+    E --> G[Result Set]
+    F --> G
+```
+
+### Components
+
+| Component | Description |
+|-----------|-------------|
+| `Timechart` AST | Represents the timechart command in the abstract syntax tree |
+| `Chart` Command Handler | Unified handler for both `chart` and `timechart` commands |
+| `SpanUnit` | Enum for time span units (millisecond, second, minute, hour, day, week, month, quarter, year) |
+| `IntervalUnit` | Enum for interval calculations including millisecond support |
+| `PlanUtils` | Utility for span unit to interval unit conversion |
+
+### Configuration
+
+| Setting | Description | Default |
+|---------|-------------|---------|
+| `timefield` | Specifies the timestamp field to use for time bucketing | `@timestamp` |
+| `span` | Time interval for bucketing (e.g., `1m`, `5m`, `1h`, `500ms`) | Required |
+
+### Syntax
+
+```
+timechart [timefield=<field>] span=<interval> <aggregation>... [by <field>]
+```
+
+#### Parameters
+
+| Parameter | Required | Description |
+|-----------|----------|-------------|
+| `timefield` | No | Custom timestamp field name (default: `@timestamp`) |
+| `span` | Yes | Time bucket interval |
+| `aggregation` | Yes | One or more aggregation functions |
+| `by` | No | Field to split results by |
+
+#### Supported Span Units
+
+| Unit | Abbreviation | Example |
+|------|--------------|---------|
+| Millisecond | `ms` | `span=500ms` |
+| Second | `s` | `span=30s` |
+| Minute | `m` | `span=5m` |
+| Hour | `h` | `span=1h` |
+| Day | `d` | `span=1d` |
+| Week | `w` | `span=1w` |
+| Month | `mon` | `span=1mon` |
+| Quarter | `q` | `span=1q` |
+| Year | `y` | `span=1y` |
+
+### Rate-Based Aggregation Functions
+
+| Function | Description | Calculation |
+|----------|-------------|-------------|
+| `per_second(field)` | Per-second rate | `sum(field) / span_in_seconds` |
+| `per_minute(field)` | Per-minute rate | `sum(field) / span_in_seconds × 60` |
+| `per_hour(field)` | Per-hour rate | `sum(field) / span_in_seconds × 3600` |
+| `per_day(field)` | Per-day rate | `sum(field) / span_in_seconds × 86400` |
+
+### Usage Examples
+
+#### Basic time-series aggregation
+
+```
+source=web_logs
+| timechart span=1h count() as requests
+```
+
+#### Rate calculation with per_second
+
+```
+source=network_logs
+| timechart span=5m per_second(bytes) as bytes_per_second
+```
+
+#### Multiple aggregations
+
+```
+source=metrics
+| timechart span=1m avg(cpu_usage), max(memory_usage), per_second(requests)
+```
+
+#### Custom timestamp field
+
+```
+source=custom_events
+| timechart timefield=event_timestamp span=1h count() by event_type
+```
+
+#### Millisecond precision for high-frequency data
+
+```
+source=trading_data
+| timechart span=100ms per_second(transactions)
+```
+
+#### Grouping by category
+
+```
+source=application_logs
+| timechart span=15m count() by log_level
+```
+
+## Limitations
+
+- `per_*` functions work exclusively with the `timechart` command
+- Variable-length spans (month/quarter/year) use dynamic calculation which may have slight performance overhead
+- The `timechart` command requires a timestamp field in the source data
+
+## Related PRs
+
+| Version | PR | Description |
+|---------|-----|-------------|
+| v3.4.0 | [#4464](https://github.com/opensearch-project/sql/pull/4464) | Add `per_second` function support |
+| v3.4.0 | [#4531](https://github.com/opensearch-project/sql/pull/4531) | Add `per_minute`, `per_hour`, `per_day` functions |
+| v3.4.0 | [#4672](https://github.com/opensearch-project/sql/pull/4672) | Support millisecond span |
+| v3.4.0 | [#4755](https://github.com/opensearch-project/sql/pull/4755) | Merge `timechart` and `chart` implementations |
+| v3.4.0 | [#4784](https://github.com/opensearch-project/sql/pull/4784) | Add `timefield` option |
+
+## References
+
+- [Issue #4350](https://github.com/opensearch-project/sql/issues/4350): PPL `per_*` aggregation function support
+- [Issue #4550](https://github.com/opensearch-project/sql/issues/4550): Millisecond span bug fix
+- [Issue #4576](https://github.com/opensearch-project/sql/issues/4576): Custom timestamp field feature request
+- [Issue #4581](https://github.com/opensearch-project/sql/issues/4581): Timechart bug fixes
+- [Issue #4582](https://github.com/opensearch-project/sql/issues/4582): Timechart bug fixes
+- [Issue #4632](https://github.com/opensearch-project/sql/issues/4632): Timechart bug fixes
+- [PPL Commands Documentation](https://docs.opensearch.org/3.0/search-plugins/sql/ppl/functions/)
+
+## Change History
+
+- **v3.4.0** (2026-01): Added `per_second`, `per_minute`, `per_hour`, `per_day` functions; millisecond span support; `timefield` option; merged `timechart` and `chart` implementations
diff --git a/docs/releases/v3.4.0/features/sql/ppl-timechart-functions.md b/docs/releases/v3.4.0/features/sql/ppl-timechart-functions.md
new file mode 100644
index 00000000..26b8ddc5
--- /dev/null
+++ b/docs/releases/v3.4.0/features/sql/ppl-timechart-functions.md
@@ -0,0 +1,162 @@
+# PPL Timechart Functions
+
+## Summary
+
+OpenSearch v3.4.0 introduces significant enhancements to the PPL `timechart` command with new rate-based aggregation functions (`per_second`, `per_minute`, `per_hour`, `per_day`), millisecond span support, custom timestamp field specification via `timefield`, and improved internal architecture by merging `timechart` and `chart` implementations.
+
+## Details
+
+### What's New in v3.4.0
+
+#### Rate-Based Aggregation Functions
+
+The `per_*` functions calculate rate-based metrics by normalizing aggregated values to specific time units:
+
+| Function | Description | Multiplier |
+|----------|-------------|------------|
+| `per_second(field)` | Per-second rate | sum / span_seconds |
+| `per_minute(field)` | Per-minute rate | sum / span_seconds × 60 |
+| `per_hour(field)` | Per-hour rate | sum / span_seconds × 3600 |
+| `per_day(field)` | Per-day rate | sum / span_seconds × 86400 |
+
+#### Implementation Approach
+
+The `per_*` functions use an **Eval Transformation** approach that rewrites the query at compile time:
+
+```
+-- Original query
+source=events | timechart span=5m per_second(packets)
+
+-- Rewritten internally to
+source=events | timechart span=5m sum(packets) as `per_second(packets)`
+             | eval `per_second(packets)` = `per_second(packets)` / 300
+```
+
+For variable-length spans (month/quarter/year), the implementation dynamically calculates bucket length:
+
+```
+-- For span=2mon, uses timestampdiff for accurate calculation
+| eval `per_second(packets)` = `per_second(packets)` /
+    timestampdiff(SECOND, @timestamp, timestampadd(MONTH, 2, @timestamp))
+```
+
+#### Custom Timestamp Field (`timefield`)
+
+Users can now specify a custom timestamp field instead of the implicit `@timestamp`:
+
+```
+source=events | timechart timefield=start_at span=1hour count() by category
+```
+
+This resolves the limitation where indexes with non-standard timestamp field names required workarounds like renaming fields.
+
+#### Millisecond Span Support
+
+Fixed the bug where millisecond spans were incorrectly converted to microseconds. The `IntervalUnit` enum now includes `MILLISECOND`, enabling accurate time bucketing for sub-second precision:
+
+```
+source=logs | timechart span=500ms count()
+```
+
+#### Architecture Improvements
+
+The `timechart` and `chart` command implementations have been merged. Since `timechart` is semantically a subset of `chart` (with row-split fixed to the timestamp field), this consolidation:
+
+- Reduces code duplication
+- Fixes several existing bugs in `timechart`
+- Simplifies maintenance
+
+### Technical Changes
+
+```mermaid
+graph TB
+    subgraph "Query Processing"
+        A[PPL Query] --> B[Parser]
+        B --> C[Timechart AST]
+        C --> D{Has per_* function?}
+        D -->|Yes| E[Rewrite to sum + eval]
+        D -->|No| F[Standard aggregation]
+        E --> G[Execute Query]
+        F --> G
+    end
+    
+    subgraph "Span Calculation"
+        H[Fixed Span] --> I[Static seconds calculation]
+        J[Variable Span] --> K[timestampdiff dynamic calculation]
+    end
+```
+
+#### New Configuration
+
+| Setting | Description | Default |
+|---------|-------------|---------|
+| `timefield` | Custom timestamp field name | `@timestamp` |
+
+### Usage Examples
+
+#### Basic per_second calculation
+
+```
+source=network_logs
+| timechart span=1m per_second(packets)
+```
+
+Result:
+```
+| @timestamp          | per_second(packets) |
+|---------------------|---------------------|
+| 2025-09-08 10:00:00 | 2.0                 |
+```
+
+#### Multiple rate functions
+
+```
+source=network_logs
+| timechart span=1m per_second(packets), per_minute(packets), per_hour(packets)
+```
+
+#### Custom timestamp field
+
+```
+source=ocsf_events
+| timechart timefield=event_time span=1h count() by category
+```
+
+#### Millisecond precision
+
+```
+source=high_frequency_logs
+| timechart span=100ms per_second(requests)
+```
+
+### Migration Notes
+
+- No breaking changes; existing `timechart` queries continue to work
+- Users with custom timestamp fields can now use `timefield` instead of `rename` workaround
+- Millisecond spans now work correctly without manual adjustments
+
+## Limitations
+
+- `per_*` functions work exclusively with the `timechart` command (due to implicit timestamp field dependency)
+- Variable-length spans (month/quarter/year) require dynamic calculation which may have slight performance overhead
+
+## Related PRs
+
+| PR | Description |
+|----|-------------|
+| [#4464](https://github.com/opensearch-project/sql/pull/4464) | Add `per_second` function support for `timechart` command |
+| [#4531](https://github.com/opensearch-project/sql/pull/4531) | Add `per_minute`, `per_hour`, `per_day` function support |
+| [#4672](https://github.com/opensearch-project/sql/pull/4672) | Support millisecond span |
+| [#4755](https://github.com/opensearch-project/sql/pull/4755) | Merge the implementation of `timechart` and `chart` |
+| [#4784](https://github.com/opensearch-project/sql/pull/4784) | Specify timestamp field with `timefield` in timechart command |
+
+## References
+
+- [Issue #4350](https://github.com/opensearch-project/sql/issues/4350): PPL `per_*` aggregation function support
+- [Issue #4550](https://github.com/opensearch-project/sql/issues/4550): Span millisecond incorrectly converted to microsecond
+- [Issue #4576](https://github.com/opensearch-project/sql/issues/4576): timechart with option to specify timestamp column
+- [PPL Commands Documentation](https://docs.opensearch.org/3.0/search-plugins/sql/ppl/functions/)
+
+## Related Feature Report
+
+- [Full feature documentation](../../../features/sql/ppl-timechart-command.md)
diff --git a/docs/releases/v3.4.0/index.md b/docs/releases/v3.4.0/index.md
index bba3660e..f1113449 100644
--- a/docs/releases/v3.4.0/index.md
+++ b/docs/releases/v3.4.0/index.md
@@ -134,6 +134,7 @@
 
 ### SQL
 
+- [PPL Timechart Functions](features/sql/ppl-timechart-functions.md) - Rate-based aggregation functions (per_second, per_minute, per_hour, per_day), millisecond span support, custom timefield option, merged timechart/chart implementation
 - [PPL Query Optimization](features/sql/ppl-query-optimization.md) - 33 enhancements including sort pushdown, aggregation optimization, distinct count approx, case-to-range queries, fillnull command, YAML explain format
 - [SQL/PPL Bugfixes](features/sql/sql-ppl-bugfixes.md) - 48 bug fixes including memory exhaustion fix, race condition fix, rex nested capture groups, filter pushdown improvements, and CVE-2025-48924
 - [SQL CI/Tests](features/sql/sql-ci-tests.md) - CI/CD improvements including Gradle 9.2.0, JDK 25, BWC test splitting, query timeouts, and maven snapshots publishing