diff --git a/datahub-graphql-core/src/main/resources/assertions.graphql b/datahub-graphql-core/src/main/resources/assertions.graphql
index 0ed264b20fe27..3014289e51178 100644
--- a/datahub-graphql-core/src/main/resources/assertions.graphql
+++ b/datahub-graphql-core/src/main/resources/assertions.graphql
@@ -213,6 +213,11 @@ enum FreshnessAssertionScheduleType {
   A scheduled based on a recurring fixed schedule which is used to compute the expected operation window. E.g. "every 24 hours".
   """
   FIXED_INTERVAL
+
+  """
+  A schedule computed based on when the assertion was last evaluated, to the current moment in time.
+  """
+  SINCE_THE_LAST_CHECK
 }
 
 """
diff --git a/datahub-graphql-core/src/main/resources/entity.graphql b/datahub-graphql-core/src/main/resources/entity.graphql
index 98d47e2cd4626..316bdd7ef5279 100644
--- a/datahub-graphql-core/src/main/resources/entity.graphql
+++ b/datahub-graphql-core/src/main/resources/entity.graphql
@@ -7658,6 +7658,16 @@ enum AssertionStdOperator {
     """
     EQUAL_TO
 
+    """
+    Value being asserted is not equal to value
+    """
+    NOT_EQUAL_TO
+
+    """
+    Value being asserted is null
+    """
+    NULL
+
     """
     Value being asserted is not null
     """
@@ -7694,12 +7704,12 @@ enum AssertionStdOperator {
     NOT_IN
 
     """
-    Value being asserted is true.
+    Value being asserted is true
     """
     IS_TRUE
 
     """
-    Value being asserted is false.
+    Value being asserted is false
     """
     IS_FALSE
 
diff --git a/metadata-models/src/main/pegasus/com/linkedin/assertion/FreshnessAssertionSchedule.pdl b/metadata-models/src/main/pegasus/com/linkedin/assertion/FreshnessAssertionSchedule.pdl
index a87342ad4f5ed..1905cb114e08c 100644
--- a/metadata-models/src/main/pegasus/com/linkedin/assertion/FreshnessAssertionSchedule.pdl
+++ b/metadata-models/src/main/pegasus/com/linkedin/assertion/FreshnessAssertionSchedule.pdl
@@ -15,7 +15,7 @@ record FreshnessAssertionSchedule {
    */
   type: enum FreshnessAssertionScheduleType {
     /**
-     * An highly configurable recurring schedule which describes the times of events described
+     * A highly configurable recurring schedule which describes the times of events described
      * by a CRON schedule, with the evaluation schedule assuming to be matching the cron schedule.
      *
      * In a CRON schedule type, we compute the look-back window to be the time between the last scheduled event
@@ -45,12 +45,31 @@ record FreshnessAssertionSchedule {
      * to be evaluated each hour, we'd compute the result as follows:
      *
      *     1. Subtract the fixed interval from the current time (Evaluation time) to compute the bounds of a fixed look-back window.
-     *     2. Verify that the target event has occurred within the CRON-interval window.
+     *     2. Verify that the target event has occurred within the look-back window.
      *     3. If the target event has occurred within the time window, then assertion passes.
      *     4. If the target event has not occurred within the time window, then the assertion fails.
      *
      */
     FIXED_INTERVAL
+    /**
+     * A stateful check that takes the last time this check ran to determine the look-back window.
+     *
+     * To compute the valid look-back- window, we start at the time the monitor last evaluated this assertion,
+     * and we end at the point in time the check is currently running.
+     *
+     * For example, let's say a Freshness assertion is of type SINCE_THE_LAST_CHECK, and the monitor is configured to
+     * run every day at 12:00am. Let's assume this assertion was last evaluated yesterday at 12:04am. We'd compute
+     * the result as follows:
+     *
+     *     1. Get the timestamp for the last run of the monitor on this assertion.
+     *     2. look_back_window_start_time = latest_monitor_run.timestampMillis [ie. 12:04a yesterday]
+     *     3. look_back_window_end_time = nowMillis [ie. 12:02a today]
+     *     4. If the target event has occurred within the window [ie. 12:04a yday to 12:02a today],
+     *        then the assertion passes.
+     *     5. If the target event has not occurred within the window, then the assertion fails.
+     *
+     */
+    SINCE_THE_LAST_CHECK
   }
 
   /**
diff --git a/metadata-models/src/main/pegasus/com/linkedin/common/CronSchedule.pdl b/metadata-models/src/main/pegasus/com/linkedin/common/CronSchedule.pdl
new file mode 100644
index 0000000000000..e59ef345186be
--- /dev/null
+++ b/metadata-models/src/main/pegasus/com/linkedin/common/CronSchedule.pdl
@@ -0,0 +1,16 @@
+namespace com.linkedin.common
+
+/**
+* Attributes defining a CRON-formatted schedule.
+*/
+record CronSchedule {
+  /**
+   * A cron-formatted execution interval, as a cron string, e.g. 1 * * * *
+   */
+  cron: string
+
+  /**
+   * Timezone in which the cron interval applies, e.g. America/Los Angeles
+   */
+  timezone: string
+}
\ No newline at end of file