diff --git a/packages/opencode/src/effect/runtime-flags.ts b/packages/opencode/src/effect/runtime-flags.ts
index 8d9d1fd97a8c..03400f39673f 100644
--- a/packages/opencode/src/effect/runtime-flags.ts
+++ b/packages/opencode/src/effect/runtime-flags.ts
@@ -52,6 +52,7 @@ export class Service extends ConfigService.Service<Service>()("@opencode/Runtime
   bashDefaultTimeoutMs: positiveInteger("OPENCODE_EXPERIMENTAL_BASH_DEFAULT_TIMEOUT_MS"),
   experimentalNativeLlm: enabledByExperimental("OPENCODE_EXPERIMENTAL_NATIVE_LLM"),
   client: Config.string("OPENCODE_CLIENT").pipe(Config.withDefault("cli")),
+  dateOverride: Config.string("OPENCODE_DATE").pipe(Config.option),
 }) {}
 
 export type Info = Context.Service.Shape<typeof Service>
diff --git a/packages/opencode/src/session/system.ts b/packages/opencode/src/session/system.ts
index 06c71fa7dbdd..b60afd07a339 100644
--- a/packages/opencode/src/session/system.ts
+++ b/packages/opencode/src/session/system.ts
@@ -1,6 +1,7 @@
-import { Context, Effect, Layer } from "effect"
+import { Context, Effect, Layer, Option } from "effect"
 
 import { InstanceState } from "@/effect/instance-state"
+import { RuntimeFlags } from "@/effect/runtime-flags"
 
 import PROMPT_ANTHROPIC from "./prompt/anthropic.txt"
 import PROMPT_DEFAULT from "./prompt/default.txt"
@@ -43,10 +44,12 @@ export const layer = Layer.effect(
   Service,
   Effect.gen(function* () {
     const skill = yield* Skill.Service
+    const flags = yield* RuntimeFlags.Service
 
     return Service.of({
       environment: Effect.fn("SystemPrompt.environment")(function* (model: Provider.Model) {
         const ctx = yield* InstanceState.context
+        const dateStr = Option.getOrElse(flags.dateOverride, () => new Date().toDateString())
         return [
           [
             `You are powered by the model named ${model.api.id}. The exact model ID is ${model.providerID}/${model.api.id}`,
@@ -56,7 +59,7 @@ export const layer = Layer.effect(
             `  Workspace root folder: ${ctx.worktree}`,
             `  Is directory a git repo: ${ctx.project.vcs === "git" ? "yes" : "no"}`,
             `  Platform: ${process.platform}`,
-            `  Today's date: ${new Date().toDateString()}`,
+            `  Today's date: ${dateStr}`,
             `</env>`,
           ].join("\n"),
         ]
@@ -79,6 +82,9 @@ export const layer = Layer.effect(
   }),
 )
 
-export const defaultLayer = layer.pipe(Layer.provide(Skill.defaultLayer))
+export const defaultLayer = layer.pipe(
+  Layer.provide(Skill.defaultLayer),
+  Layer.provide(RuntimeFlags.defaultLayer),
+)
 
 export * as SystemPrompt from "./system"
diff --git a/packages/opencode/test/session/system.test.ts b/packages/opencode/test/session/system.test.ts
index 1cf9026725ea..d216ea947a5d 100644
--- a/packages/opencode/test/session/system.test.ts
+++ b/packages/opencode/test/session/system.test.ts
@@ -2,6 +2,7 @@ import { describe, expect } from "bun:test"
 import { Effect, Layer } from "effect"
 import type { Agent } from "../../src/agent/agent"
 import { NamedError } from "@opencode-ai/core/util/error"
+import { RuntimeFlags } from "../../src/effect/runtime-flags"
 import { Skill } from "../../src/skill"
 import { Permission } from "../../src/permission"
 import { SystemPrompt } from "../../src/session/system"
@@ -53,6 +54,7 @@ const it = testEffect(
         }),
       ),
     ),
+    Layer.provide(RuntimeFlags.defaultLayer),
   ),
 )