Same agent. Different risk by the clock.

Time-based AI agent policies use OPA’s time builtins so tool permissions follow business hours, lunch blocks, and change freezes. Official reference: OPA time builtins. You already have authorize and Terraform attach. This post adds the clock.

Key takeaways

  • Drive windows from input.timestamp, not only wall clock, so tests stay fixed.
  • Allow-list hours and freeze windows are different postures: one denies everything off-hours; the other denies change actions on weekdays.
  • Put fixed timestamps in opa test fixtures so CI does not depend on wall clock.

UTC allow morning deny lunch allow afternoon deny off-hours

Parse the factsheet clock. Do not invent a second time source in Rego if the platform already sent one.

Architecture: time is an input field, not a prompt

Time-based authorization is still the same PDP/PEP loop from authorize. The only change is which factsheet fields Rego reads.

Source Use
input.timestamp Platform factsheet clock (RFC3339). Prefer this for business hours and freezes so opa test fixtures stay fixed.
time.parse_rfc3339_ns Turn that string into nanoseconds for time.clock / time.weekday.
time.now_ns Wall clock at evaluate time. Useful for skew or staleness checks; avoid as the only clock in unit tests.

Two postures share that architecture:

  1. Allow-list hours (default allow := false): every tool is denied unless the hour falls in a window.
  2. Change freeze (default allow := true): most tools pass; change actions fail during weekday business hours.

Both are logic policies. Attach them like any other Rego. They do not need a special runtime mode.

Why prompts fail at “only during business hours”

A persona line that says “do not deploy after 5pm” is advice. An agent investigating an outage at 02:00 will still reach for kubectl apply if that tool is available. Time has to be a field on the enforcement input, not a hope in the system prompt.

Tool-hours allowlist

Opt-in pattern from solutions tool-hours-allowlist.rego: default deny; allow only in morning and afternoon UTC windows; lunch is blocked.

package policy

import rego.v1

default allow := false

factsheet_ts_ns := time.parse_rfc3339_ns(ts) if {
  ts := object.get(input, "timestamp", "")
  ts != ""
}

hour := time.clock(ns)[0] if {
  ns := factsheet_ts_ns
}

in_morning_allow_window if {
  hour >= 9
  hour < 12
}

in_afternoon_allow_window if {
  hour >= 14
  hour < 17
}

allow if {
  in_morning_allow_window
}

allow if {
  in_afternoon_allow_window
}

Lab: samples/aiden-opa-policies/03-business-hours/ with tests for mid-morning, lunch, afternoon, and evening.

Change freeze window

Different question: during Mon-Fri 09:00-17:59 UTC, block change actions, not every tool.

package policy

import rego.v1

default allow := true

change_actions := {"deploy", "apply", "release", "helm_upgrade", "rollout"}

during_business_hours if {
  ns := time.parse_rfc3339_ns(input.timestamp)
  time.weekday(ns) in {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday"}
  hour := time.clock(ns)[0]
  hour >= 9
  hour < 18
}

allow := false if {
  during_business_hours
  input.tool.arguments.action in change_actions
}

allow := false if {
  during_business_hours
  endswith(input.tool.name, "_execute_command")
  contains(lower(input.tool.arguments.command), "kubectl apply")
}

Lab: samples/aiden-opa-policies/04-freeze-and-hitl/freeze/.

How do you verify?

cd samples/aiden-opa-policies
opa test 03-business-hours
opa test 04-freeze-and-hitl/freeze

Use fixed RFC3339 strings in tests (2026-07-16T13:15:00Z for lunch). Do not call time.now_ns inside unit tests if you want green CI at midnight.

Attach either policy with the Terraform pattern from the previous post: type = "logic", file(...) for Rego, then sg_agent_policy_attachment.

Next: when to hard deny versus ask a human. Guardrails and human approval for AI agent tool calls.

Freeze windows belong in Rego, not in a sticky note on the on-call channel. Labs: samples/aiden-opa-policies. To attach on a live tenant: free account then personal access token.

Free to try: create an account, then create a personal access token for Terraform. StackGen / Aiden: app.stackgen.com.

Previous: Manage AI agent policies as code with Terraform. Next: Guardrails and human approval for AI agent tool calls.

FAQ

How do you write time-based AI agent policies with OPA?

Parse input.timestamp with time.parse_rfc3339_ns, then use time.clock and time.weekday. Allow-list windows or deny change actions during freeze hours.

Should policies call time.now_ns or use the factsheet timestamp?

Prefer input.timestamp from the platform factsheet so evaluations and opa test fixtures stay deterministic. time.now_ns is useful for skew checks when you also have a trusted clock.

What is a tool-hours allowlist?

A logic policy that defaults to deny and only sets allow inside named UTC windows, for example morning and afternoon with a lunch gap.

What is a change freeze policy?

A policy that denies deploy, apply, and similar actions during weekday business hours while still allowing reads and status.