diff --git a/changelog/27350.txt b/changelog/27350.txt
new file mode 100644
index 000000000000..ddf95bc0052b
--- /dev/null
+++ b/changelog/27350.txt
@@ -0,0 +1,4 @@
+```release-note:change
+activity log: Deprecated the field "default_report_months". Instead, the billing start time will be used to determine the start time
+when querying the activity log endpoints.
+```
\ No newline at end of file
diff --git a/vault/activity_log.go b/vault/activity_log.go
index 1c98d3122dd2..9aab39684be0 100644
--- a/vault/activity_log.go
+++ b/vault/activity_log.go
@@ -1993,6 +1993,7 @@ func (a *ActivityLog) modifyResponseMonths(months []*ResponseMonth, start time.T
 type activityConfig struct {
 	// DefaultReportMonths are the default number of months that are returned on
 	// a report. The zero value uses the system default of 12.
+	// Deprecated: This field was removed in favor of using different default startTime and endTime values
 	DefaultReportMonths int `json:"default_report_months"`
 
 	// RetentionMonths defines the number of months we want to retain data. The
diff --git a/vault/activity_log_test.go b/vault/activity_log_test.go
index 733b6dc10eb0..620ce6f63493 100644
--- a/vault/activity_log_test.go
+++ b/vault/activity_log_test.go
@@ -1016,7 +1016,6 @@ func TestActivityLog_API_ConfigCRUD_Census(t *testing.T) {
 	}
 
 	expected := map[string]interface{}{
-		"default_report_months":    12,
 		"retention_months":         56,
 		"enabled":                  "enable",
 		"queries_available":        false,
diff --git a/vault/logical_system_activity.go b/vault/logical_system_activity.go
index f7607a191cd0..8eeeb24ad34d 100644
--- a/vault/logical_system_activity.go
+++ b/vault/logical_system_activity.go
@@ -108,6 +108,7 @@ func (b *SystemBackend) rootActivityPaths() []*framework.Path {
 					Type:        framework.TypeInt,
 					Default:     12,
 					Description: "Number of months to report if no start date specified.",
+					Deprecated:  true,
 				},
 				"retention_months": {
 					Type:        framework.TypeInt,
@@ -328,7 +329,6 @@ func (b *SystemBackend) handleActivityConfigRead(ctx context.Context, req *logic
 
 	return &logical.Response{
 		Data: map[string]interface{}{
-			"default_report_months":    config.DefaultReportMonths,
 			"retention_months":         config.RetentionMonths,
 			"enabled":                  config.Enabled,
 			"queries_available":        qa,
@@ -358,8 +358,8 @@ func (b *SystemBackend) handleActivityConfigUpdate(ctx context.Context, req *log
 
 	{
 		// Parse the default report months
-		if defaultReportMonthsRaw, ok := d.GetOk("default_report_months"); ok {
-			config.DefaultReportMonths = defaultReportMonthsRaw.(int)
+		if _, ok := d.GetOk("default_report_months"); ok {
+			warnings = append(warnings, fmt.Sprintf("default_report_months is deprecated: defaulting to billing start time"))
 		}
 
 		if config.DefaultReportMonths <= 0 {
diff --git a/website/content/api-docs/system/internal-counters.mdx b/website/content/api-docs/system/internal-counters.mdx
index d19e4a54171c..69be0d763d3a 100644
--- a/website/content/api-docs/system/internal-counters.mdx
+++ b/website/content/api-docs/system/internal-counters.mdx
@@ -1095,7 +1095,8 @@ The `/sys/internal/counters/config` endpoint is used to configure logging of act
 
 ### Parameters
 
-- `default_report_months` `(integer: 12)` - The number of months to report if no `start_time` is specified in a query.
+- `default_report_months` `(integer: 12)` - **DEPRECATED** The number
+   of months to report if no `start_time` is specified in a query.
 - `enabled` `(string: enable, disable, default)` - Enable or disable counting of client activity. When set to `default`, the client
   counts are enabled on Enterprise builds and disabled on community builds. Disabling the feature during the middle of a month will
   discard any data recorded for that month, but does not delete previous months.
@@ -1108,7 +1109,6 @@ Any missing parameters are left at their existing value.
 ```json
 {
   "enabled": "enable",
-  "default_report_months": 3,
   "retention_months": 54
 }
 ```
@@ -1153,7 +1153,6 @@ $ curl \
   "lease_duration": 0,
   "renewable": false,
   "data": {
-    "default_report_months": 12,
     "enabled": "default-enabled",
     "queries_available": true,
     "retention_months": 48,
diff --git a/website/content/docs/upgrading/upgrade-to-1.18.x.mdx b/website/content/docs/upgrading/upgrade-to-1.18.x.mdx
new file mode 100644
index 000000000000..d111c6c9c944
--- /dev/null
+++ b/website/content/docs/upgrading/upgrade-to-1.18.x.mdx
@@ -0,0 +1,38 @@
+---
+layout: docs
+page_title: Upgrade to Vault 1.18.x - Guides
+description: |-
+  Deprecations, important or breaking changes, and remediation recommendations
+  for anyone upgrading to 1.18.x from Vault 1.17.x.
+---
+
+# Overview
+
+The Vault 1.18.x upgrade guide contains information on deprecations, important
+or breaking changes, and remediation recommendations for anyone upgrading from
+Vault 1.17. **Please read carefully**.
+
+## Important changes
+
+### Activity Log Changes
+
+The field `default_report_months` can no longer be configured or read. Any previously set values
+will be ignored by the system.
+
+
+Attempts to modify `default_report_months` through the
+[/sys/internal/counters/config](/vault/api-docs/system/internal-counters#update-the-client-count-configuration)
+endpoint, will result in the following warning from Vault:
+
+<CodeBlockConfig hideClipboard>
+
+```shell-session
+
+WARNING! The following warnings were returned from Vault:
+
+  * default_report_months is deprecated: defaulting to billing start time
+
+
+```
+
+</CodeBlockConfig>
diff --git a/website/data/docs-nav-data.json b/website/data/docs-nav-data.json
index 340e86a49868..9282b5e9767c 100644
--- a/website/data/docs-nav-data.json
+++ b/website/data/docs-nav-data.json
@@ -2325,6 +2325,10 @@
         "title": "Upgrade to Raft WAL",
         "path": "upgrading/raft-wal"
       },
+      {
+        "title": "Upgrade to 1.18.x",
+        "path": "upgrading/upgrade-to-1.18.x"
+      },
       {
         "title": "Upgrade to 1.17.x",
         "path": "upgrading/upgrade-to-1.17.x"