diff --git a/.idea/cmake.xml b/.idea/cmake.xml
new file mode 100644
index 00000000..b0a4921a
--- /dev/null
+++ b/.idea/cmake.xml
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="CMakeSharedSettings">
+    <configurations>
+      <configuration PROFILE_NAME="Debug Test" ENABLED="true" CONFIG_NAME="Debug" GENERATION_OPTIONS="-DFSFW_BUILD_TESTS=ON -DFSFW_OSAL=host" />
+    </configurations>
+  </component>
+</project>
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index b4e9ab8e..9f5bb9bb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Fixes
 
+- The `PusTmCreator` API only accepted 255 bytes of source data. It can now accept source
+  data with a size limited only by the size of `size_t`.
 - Important bugfix in CFDP PDU header format: The entity length field and the transaction sequence
   number fields stored the actual length of the field instead of the length minus 1 like specified
   in the CFDP standard.
@@ -28,6 +30,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - `PusTmZcWriter` now exposes API to set message counter field.
 
 ## Changed
+
+- HK generation is now countdown based.
 - Bump ETL version to 20.35.14
   https://egit.irs.uni-stuttgart.de/fsfw/fsfw/pulls/748
 - Renamed `PCDU_2` subsystem ID to `POWER_SWITCH_IF`.
diff --git a/src/fsfw/coordinates/Sgp4Propagator.cpp b/src/fsfw/coordinates/Sgp4Propagator.cpp
index e79ffef5..62c2670e 100644
--- a/src/fsfw/coordinates/Sgp4Propagator.cpp
+++ b/src/fsfw/coordinates/Sgp4Propagator.cpp
@@ -166,9 +166,9 @@ ReturnValue_t Sgp4Propagator::propagate(double* position, double* velocity, time
   timeval timeSinceEpoch = time - epoch;
   double minutesSinceEpoch = timeSinceEpoch.tv_sec / 60. + timeSinceEpoch.tv_usec / 60000000.;
 
-  double yearsSinceEpoch = minutesSinceEpoch / 60 / 24 / 365;
+  double monthsSinceEpoch = minutesSinceEpoch / 60 / 24 / 30;
 
-  if ((yearsSinceEpoch > 1) || (yearsSinceEpoch < -1)) {
+  if ((monthsSinceEpoch > 1) || (monthsSinceEpoch < -1)) {
     return TLE_TOO_OLD;
   }
 
diff --git a/src/fsfw/housekeeping/PeriodicHousekeepingHelper.cpp b/src/fsfw/housekeeping/PeriodicHousekeepingHelper.cpp
index b39e45c3..ec34330a 100644
--- a/src/fsfw/housekeeping/PeriodicHousekeepingHelper.cpp
+++ b/src/fsfw/housekeeping/PeriodicHousekeepingHelper.cpp
@@ -3,6 +3,7 @@
 #include <cmath>
 
 #include "fsfw/datapoollocal/LocalPoolDataSetBase.h"
+#include "fsfw/serviceinterface.h"
 
 PeriodicHousekeepingHelper::PeriodicHousekeepingHelper(LocalPoolDataSetBase* owner)
     : owner(owner) {}
@@ -10,51 +11,29 @@ PeriodicHousekeepingHelper::PeriodicHousekeepingHelper(LocalPoolDataSetBase* own
 void PeriodicHousekeepingHelper::initialize(float collectionInterval,
                                             dur_millis_t minimumPeriodicInterval) {
   this->minimumPeriodicInterval = minimumPeriodicInterval;
-  collectionIntervalTicks = intervalSecondsToIntervalTicks(collectionInterval);
-  /* This will cause a checkOpNecessary call to be true immediately. I think it's okay
-      if a HK packet is generated immediately instead of waiting one generation cycle. */
-  internalTickCounter = collectionIntervalTicks;
+  changeCollectionInterval(collectionInterval);
 }
 
 float PeriodicHousekeepingHelper::getCollectionIntervalInSeconds() const {
-  return intervalTicksToSeconds(collectionIntervalTicks);
+  return collectionInterval;
 }
 
 bool PeriodicHousekeepingHelper::checkOpNecessary() {
-  if (internalTickCounter >= collectionIntervalTicks) {
-    internalTickCounter = 1;
+  if (hkGenerationCd.hasTimedOut()) {
+    hkGenerationCd.resetTimer();
     return true;
   }
-  internalTickCounter++;
   return false;
 }
 
-uint32_t PeriodicHousekeepingHelper::intervalSecondsToIntervalTicks(
-    float collectionIntervalSeconds) {
-  if (owner == nullptr) {
-    return 0;
-  }
-
-  /* Avoid division by zero */
-  if (minimumPeriodicInterval == 0) {
-    /* Perform operation each cycle */
-    return 1;
-
-  } else {
-    dur_millis_t intervalInMs = collectionIntervalSeconds * 1000;
-    uint32_t divisor = minimumPeriodicInterval;
-    uint32_t ticks = std::ceil(static_cast<float>(intervalInMs) / divisor);
-
-    return ticks;
-  }
-}
-
-float PeriodicHousekeepingHelper::intervalTicksToSeconds(uint32_t collectionInterval) const {
-  /* Number of ticks times the minimum interval is in milliseconds, so we divide by 1000 to get
-  the value in seconds */
-  return static_cast<float>(collectionInterval * minimumPeriodicInterval / 1000.0);
-}
-
 void PeriodicHousekeepingHelper::changeCollectionInterval(float newIntervalSeconds) {
-  collectionIntervalTicks = intervalSecondsToIntervalTicks(newIntervalSeconds);
+  uint32_t intervalMs = newIntervalSeconds * 1000;
+  if (newIntervalSeconds <= 0) {
+    intervalMs = minimumPeriodicInterval;
+    newIntervalSeconds = static_cast<float>(minimumPeriodicInterval) / 1000.0;
+  }
+  collectionInterval = newIntervalSeconds;
+  hkGenerationCd.setTimeout(intervalMs);
+  // We want an immediate HK packet at the start, so time out the generation CD immediately.
+  hkGenerationCd.timeOut();
 }
diff --git a/src/fsfw/housekeeping/PeriodicHousekeepingHelper.h b/src/fsfw/housekeeping/PeriodicHousekeepingHelper.h
index 1ec0febf..1586649c 100644
--- a/src/fsfw/housekeeping/PeriodicHousekeepingHelper.h
+++ b/src/fsfw/housekeeping/PeriodicHousekeepingHelper.h
@@ -4,6 +4,7 @@
 #include <cstdint>
 
 #include "fsfw/timemanager/Clock.h"
+#include "fsfw/timemanager/Countdown.h"
 
 class LocalPoolDataSetBase;
 
@@ -19,13 +20,10 @@ class PeriodicHousekeepingHelper {
 
  private:
   LocalPoolDataSetBase* owner = nullptr;
-
-  uint32_t intervalSecondsToIntervalTicks(float collectionIntervalSeconds);
-  float intervalTicksToSeconds(uint32_t collectionInterval) const;
+  Countdown hkGenerationCd;
+  float collectionInterval = 0.0;
 
   dur_millis_t minimumPeriodicInterval = 0;
-  uint32_t internalTickCounter = 1;
-  uint32_t collectionIntervalTicks = 0;
 };
 
 #endif /* FSFW_HOUSEKEEPING_PERIODICHOUSEKEEPINGHELPER_H_ */
diff --git a/src/fsfw/tmtcpacket/pus/tm/PusTmCreator.h b/src/fsfw/tmtcpacket/pus/tm/PusTmCreator.h
index 626d873e..19922b2f 100644
--- a/src/fsfw/tmtcpacket/pus/tm/PusTmCreator.h
+++ b/src/fsfw/tmtcpacket/pus/tm/PusTmCreator.h
@@ -40,7 +40,7 @@ struct PusTmParams {
               size_t dataLen)
       : secHeader(service, subservice, timeStamper), adapter(data, dataLen), sourceData(&adapter) {}
   PusTmSecHeader secHeader;
-  SerialBufferAdapter<uint8_t> adapter;
+  SerialBufferAdapter<size_t> adapter;
   const SerializeIF* sourceData = nullptr;
 };
 
diff --git a/unittests/CatchFactory.cpp b/unittests/CatchFactory.cpp
index 37c62f94..589e6887 100644
--- a/unittests/CatchFactory.cpp
+++ b/unittests/CatchFactory.cpp
@@ -32,7 +32,7 @@ void Factory::produceFrameworkObjects(void* args) {
   setStaticFrameworkObjectIds();
   new EventManager(objects::EVENT_MANAGER, 120);
   new HealthTable(objects::HEALTH_TABLE);
-  new InternalErrorReporter(objects::INTERNAL_ERROR_REPORTER);
+  new InternalErrorReporter(objects::INTERNAL_ERROR_REPORTER, 20, false, 1.0);
 
   {
     PoolManager::LocalPoolConfig poolCfg = {{100, 16}, {50, 32}, {25, 64}, {15, 128}, {5, 1024}};