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+- Start Date: 2024-07-29
+- RFC PR: [amaranth-lang/rfcs#66](https://github.com/amaranth-lang/rfcs/pull/66)
+- Amaranth Issue: [amaranth-lang/amaranth#0000](https://github.com/amaranth-lang/amaranth/issues/1469)
+
+# Simulation time
+
+## Summary
+[summary]: #summary
+
+Add a type for representing time durations (including clock periods) and simulator methods to query the elapsed time.
+
+## Motivation
+[motivation]: #motivation
+
+In [RFC #36](0036-async-testbench-functions.md), there were a plan to introduce a `.time()` method to query the current simulation time, but it got deferred due to the lack of a suitable return type.
+
+Internally simulation time is an integer number of femtoseconds, but exposing that directly is not ergonomic and making it a `float` of seconds sacrifices precision the longer a simulation runs.
+
+Also, to quote @whitequark: Time is not a number.
+
+## Guide- and reference-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+A new type `Period` is introduced.
+It is exported from `amaranth.hdl` and also re-exported from `amaranth.sim`.
+It is immutable and has a constructor accepting at most one named argument, giving the following valid forms of construction:
+- `Period()`
+  - Constructs a zero period.
+- `Period(s: numbers.Real)`
+- `Period(ms: numbers.Real)`
+- `Period(us: numbers.Real)`
+- `Period(ns: numbers.Real)`
+- `Period(ps: numbers.Real)`
+- `Period(fs: numbers.Real)`
+  - The argument will be scaled according to its SI prefix and used to calculate the closest integer femtosecond representation.
+- `Period(Hz: numbers.Real)`
+- `Period(kHz: numbers.Real)`
+- `Period(MHz: numbers.Real)`
+- `Period(GHz: numbers.Real)`
+  - The argument will be scaled according to its SI prefix and its reciprocal used to calculate the closest integer femtosecond representation.
+  - A value of zero will raise `ZeroDivisionError`.
+  - A negative value will raise `ValueError`.
+
+To convert it back to a number, the following properties are available:
+- `.seconds -> float`
+- `.milliseconds -> float`
+- `.microseconds -> float`
+- `.nanoseconds -> float`
+- `.picoseconds -> float`
+- `.femtoseconds -> int`
+
+To calculate the reciprocal frequency, the following properties are available:
+- `.hertz -> float`
+- `.kilohertz -> float`
+- `.megahertz -> float`
+- `.gigahertz -> float`
+  - Accessing these properties when the period is zero will raise `ZeroDivisionError`.
+  - Accessing these properties when the period is negative will raise `ValueError`.
+
+The following operators are defined:
+- `.__lt__(other: Period) -> bool`
+- `.__le__(other: Period) -> bool`
+- `.__eq__(other: Period) -> bool`
+- `.__ne__(other: Period) -> bool`
+- `.__gt__(other: Period) -> bool`
+- `.__ge__(other: Period) -> bool`
+- `.__hash__() -> int`
+- `.__bool__() -> bool`
+- `.__neg__() -> Period`
+- `.__pos__() -> Period`
+- `.__abs__() -> Period`
+- `.__add__(other: Period) -> Period`
+- `.__sub__(other: Period) -> Period`
+- `.__mul__(other: numbers.Real) -> Period`
+- `.__rmul__(other: numbers.Real) -> Period`
+- `.__truediv__(other: numbers.Real) -> Period`
+- `.__truediv__(other: Period) -> float`
+- `.__floordiv__(other: Period) -> int`
+- `.__mod__(other: Period) -> Period`
+  - Operators on `Period`s are performed on the underlying femtosecond values.
+  - Operators involving `numbers.Real` operands have the result rounded back to the closest integer femtosecond representation.
+  - Operators given unsupported operand combinations will return `NotImplemented`.
+- `.__str__() -> str`
+  - Equivalent to `.__format__("")`
+- `.__format__(format_spec: str) -> str`
+  - The format specifier format is `[width][.precision][ ][unit]`.
+  - An invalid format specifier raises `ValueError`.
+  - If `width` is specified, the string is left-padded with space to at least the requested width.
+  - If `precision` is specified, the requested number of decimal digits will be emitted.
+    Otherwise, duration units will emit as many digits required for an exact result, while frequency units will defer to default `float` formatting.
+  - If a space is present in the format specifier, the formatted string will have a space between the number and the unit.
+  - `unit` can be specified as any of the argument names accepted by the constructor.
+    If a unit is not specified, the largest duration unit that has a nonzero integer part is used.
+    Formatting frequency units have the same restrictions and exception behavior as accessing frequency properties.
+
+`SimulatorContext` have an `.elapsed_time() -> Period` method added that returns the elapsed time since start of simulation.
+
+These methods that has a `period` argument currently taking seconds as a `float` are updated to take a `Period`:
+- `Simulator.add_clock()`
+- `SimulatorContext.delay()`
+
+These methods that has a `frequency` argument currently taking Hz as a `float` are updated to take a `Period`:
+- `ResourceManager.add_clock_constraint()`
+- `Clock.__init__()`
+
+The ability to pass seconds or Hz directly as a `float` is deprecated and removed in a future Amaranth version.
+
+The `Clock.period` property is updated to return a `Period` and the `Clock.frequency` property is deprecated.
+
+Consequently, `Platform.default_clk_frequency()` is also deprecated and replaced with a new method `Platform.default_clk_period() -> Period`.
+
+
+## Drawbacks
+[drawbacks]: #drawbacks
+
+- Deprecating being able to pass seconds or Hz directly as a `float` creates churn.
+
+## Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- `.add_clock()` is usually passed the reciprocal of a frequency, so being able to construct a `Period` from a frequency instead of a duration is useful.
+  - We could add a separate `Frequency` type, but in practice it would likely almost exclusively be used to calculate the reciprocal `Period`.
+
+- Accepting a `numbers.Real` when we're converting from a number lets us pass in any standard suitable number type.
+
+- The supported set of operators are the ones that have meaningful and useful semantics:
+  - Comparing, adding and subtracting time periods makes sense.
+  - Multiplying a time period with or dividing it by a real number scales the time period.
+  - Dividing time periods gives the ratio between them.
+  - Modulo of time periods is the remainder and thus still a time period.
+    Potentially useful to calculate the phase offset between a timestamp and a clock period.
+  - Reflected operators that only accept `Period` operands are redundant and omitted.
+
+- `.elapsed_time()` is only available from within a testbench/process, where it is well defined.
+
+- Instead of named constructor arguments, we could use a classmethod for each SI prefix.
+  This was proposed in an earlier revision of this RFC.
+
+- Instead of returning `float`, we could return `fractions.Fraction` to ensure no precision loss when the number of femtoseconds is larger than the `float` significand.
+  This was proposed in an earlier revision of this RFC.
+  - Rounding to integer femtoseconds is already lossy and a further precision loss from converting to a `float` is negligible in most real world use cases.
+    For any applications requiring an exact conversion of a `Period`, the `.femtoseconds` property is always exact.
+
+## Prior art
+[prior-art]: #prior-art
+
+None.
+
+## Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+## Future possibilities
+[future-possibilities]: #future-possibilities
+
+None.