diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index ee934a6..498742d 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -32,6 +32,7 @@ Provides reusable helpers sourced by scripts:
 | `configure_gh_auth [scope_hint]` | Bridge GITHUB_TOKEN→GH_TOKEN or verify gh auth session |
 | `validate_github_token [bearer]` | Verify GITHUB_TOKEN via /user endpoint |
 | `validate_slug <value> <label>` | Reject values with non-alphanumeric/hyphen/underscore chars |
+| `is_bsd_date` | Returns 0 if `date` is BSD-style (macOS), 1 for GNU (Linux); branch between `date -v` and `date -d` syntax |
 | `gh_api <path> [--api-version V] [curl args...]` | Bearer-auth REST helper with 5-retry rate-limit handling; optional `--api-version` overrides the default `2022-11-28` header; returns literal `__404__` or `__422__` for those HTTP statuses — callers must check for these sentinels |
 | `gh_api_paginate <path> [filter] [version]` | Paginated REST helper, follows Link headers, streams items; returns silently with empty output on 404/422 |
 | `get_enterprise_orgs` | Three-tier enterprise org resolver (REST → GraphQL → /user/orgs) |
diff --git a/AGENTS.md b/AGENTS.md
index a5dcf98..853f283 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -93,7 +93,7 @@ bats tests/test_common.bats
 
 | File | What it covers |
 |------|----------------|
-| `tests/test_common.bats` | `lib/github-common.sh` — pure-logic functions (`validate_slug`, `require_env_var`, `require_command`, `err`, `configure_gh_auth`, `validate_token`, `get_repo_page_count`) and API helpers (`gh_api` sentinels, `gh_api_paginate`) |
+| `tests/test_common.bats` | `lib/github-common.sh` — pure-logic functions (`validate_slug`, `require_env_var`, `require_command`, `err`, `configure_gh_auth`, `validate_token`, `get_repo_page_count`, `is_bsd_date`) and API helpers (`gh_api` sentinels, `gh_api_paginate`) |
 | `tests/test_script_validation.bats` | Every script — missing required env vars exit 1, invalid CLI args exit 1, `--help` exits 0, script-specific enum/allowlist validation |
 | `tests/mock_curl.sh` | Universal drop-in curl mock (used by both test files); response data via env vars `MOCK_CURL_CODE`, `MOCK_CURL_BODY`, `MOCK_CURL_LINK` |
 
diff --git a/README.md b/README.md
index 626a50d..78054a5 100644
--- a/README.md
+++ b/README.md
@@ -937,6 +937,7 @@ All scripts can leverage a shared utility library for common operations like val
 - `validate_token <VAR_NAME> [bearer]` — Validate GitHub token by calling `/user` endpoint
 - `validate_github_token [bearer]` — Convenience wrapper for `GITHUB_TOKEN` validation
 - `validate_slug <value> <label>` — Exit if value contains characters other than alphanumeric, hyphen, or underscore
+- `is_bsd_date` — Return 0 if the system `date` is BSD-style (macOS), 1 for GNU (Linux); use to branch between `date -v` and `date -d` syntax
 
 **Auth helpers:**
 - `configure_gh_auth [scope_hint]` — Bridge `GITHUB_TOKEN→GH_TOKEN` for scripts that use the `gh` CLI, or verify an active `gh` auth session if no token is set
diff --git a/lib/github-common.sh b/lib/github-common.sh
index 5efec66..19f9859 100755
--- a/lib/github-common.sh
+++ b/lib/github-common.sh
@@ -18,6 +18,7 @@
 #   validate_github_token [bearer]            — verify GITHUB_TOKEN via /user endpoint
 #   validate_token <VAR_NAME>                 — verify a secondary token variable
 #   validate_slug <value> [label]             — exit if value contains unsafe chars
+#   is_bsd_date                                — 0 if `date` is BSD-style (macOS), 1 for GNU (Linux)
 #   gh_api <path|url> [--api-version V] [curl args…] — Bearer-auth REST helper with retry;
 #                                               returns "__404__"/"__422__" (exit 0) for those codes
 #   gh_api_paginate <path> [filter] [version] — paginated REST, follows Link headers;
@@ -153,6 +154,15 @@ get_repo_page_count() {
   echo "${pages:-1}"
 }
 
+###
+## is_bsd_date
+## Returns 0 if the system `date` is BSD-style (macOS), 1 for GNU (Linux).
+## Use to branch between `date -v` and `date -d` syntax.
+###
+is_bsd_date() {
+  date -v-1d > /dev/null 2>&1
+}
+
 ###
 ## validate_slug <value> <label>
 ## Exits with status 1 if the value contains characters other than
diff --git a/org-admin/github-archive-old-repos/github-archive-old-repos.sh b/org-admin/github-archive-old-repos/github-archive-old-repos.sh
index 4dcedf5..2860895 100755
--- a/org-admin/github-archive-old-repos/github-archive-old-repos.sh
+++ b/org-admin/github-archive-old-repos/github-archive-old-repos.sh
@@ -54,7 +54,7 @@ trap cleanup EXIT
 ###
 calculate_cutoff_date() {
     # Using 'date' compatible with both GNU (Linux) and BSD (macOS)
-    if date -v-1d > /dev/null 2>&1; then
+    if is_bsd_date; then
         # BSD date (macOS)
         CUTOFF_DATE=$(date -u -v-${YEARS_THRESHOLD}y +"%Y-%m-%dT%H:%M:%SZ")
     else
@@ -98,7 +98,7 @@ fetch_old_repos() {
 
             if [[ "$REPO_UPDATEDAT" < "$cutoff_date" ]]; then
                 local UPDATED_EPOCH CURRENT_EPOCH DAYS_SINCE
-                if date -v-1d > /dev/null 2>&1; then
+                if is_bsd_date; then
                     UPDATED_EPOCH=$(date -jf "%Y-%m-%dT%H:%M:%SZ" "$REPO_UPDATEDAT" +%s 2>/dev/null || echo "0")
                 else
                     UPDATED_EPOCH=$(date -d "$REPO_UPDATEDAT" +%s 2>/dev/null || echo "0")
diff --git a/reporting/github-copilot-budget-advisor/github-copilot-budget-advisor.sh b/reporting/github-copilot-budget-advisor/github-copilot-budget-advisor.sh
index 7110d46..2e30557 100755
--- a/reporting/github-copilot-budget-advisor/github-copilot-budget-advisor.sh
+++ b/reporting/github-copilot-budget-advisor/github-copilot-budget-advisor.sh
@@ -124,7 +124,7 @@ _CURR_YEAR=$(date +%Y)
 _CURR_MONTH=$(( 10#$(date +%m) ))   # strip leading zero for arithmetic
 _CURR_DAY=$(( 10#$(date +%d) ))
 
-if date -v-1m +%Y >/dev/null 2>&1; then
+if is_bsd_date; then
     # BSD date (macOS)
     _PREV_YEAR=$(date -v-1m +%Y)
     _PREV_MONTH=$(( 10#$(date -v-1m +%m) ))
diff --git a/tests/test_common.bats b/tests/test_common.bats
index 11cdc3e..c00bb17 100644
--- a/tests/test_common.bats
+++ b/tests/test_common.bats
@@ -91,6 +91,17 @@ _mock_curl() {
   [ "$status" -eq 1 ]
 }
 
+# ─── is_bsd_date ──────────────────────────────────────────────────────────────
+
+@test "is_bsd_date: matches whether 'date -v' is supported on this system" {
+  run bash -c "GITHUB_TOKEN=x source '${LIB_PATH}' 2>/dev/null; is_bsd_date"
+  if date -v-1d > /dev/null 2>&1; then
+    [ "$status" -eq 0 ]
+  else
+    [ "$status" -eq 1 ]
+  fi
+}
+
 # ─── require_env_var ──────────────────────────────────────────────────────────
 
 @test "require_env_var: exits 1 when variable is unset" {