--- a/dom/file/BaseBlobImpl.cpp
+++ b/dom/file/BaseBlobImpl.cpp
@@ -59,17 +59,18 @@ BaseBlobImpl::GetType(nsAString& aType)
   aType = mContentType;
 }
 
 int64_t
 BaseBlobImpl::GetLastModified(ErrorResult& aRv)
 {
   MOZ_ASSERT(mIsFile, "Should only be called on files");
   if (IsDateUnknown()) {
-    mLastModificationDate = nsRFPService::ReduceTimePrecisionAsUSecs(PR_Now());
+    mLastModificationDate = nsRFPService::ReduceTimePrecisionAsUSecs(PR_Now(), 0);
+    // mLastModificationDate is an absolute timestamp so we supply a zero context mix-in
   }
 
   return mLastModificationDate / PR_USEC_PER_MSEC;
 }
 
 void
 BaseBlobImpl::SetLastModified(int64_t aLastModified)
 {

--- a/dom/file/MultipartBlobImpl.cpp
+++ b/dom/file/MultipartBlobImpl.cpp
@@ -264,17 +264,18 @@ MultipartBlobImpl::SetLengthAndModifiedD
   mLength = totalLength;
 
   if (mIsFile) {
     // We cannot use PR_Now() because bug 493756 and, for this reason:
     //   var x = new Date(); var f = new File(...);
     //   x.getTime() < f.dateModified.getTime()
     // could fail.
     mLastModificationDate = nsRFPService::ReduceTimePrecisionAsUSecs(
-      lastModifiedSet ? lastModified * PR_USEC_PER_MSEC : JS_Now());
+      lastModifiedSet ? lastModified * PR_USEC_PER_MSEC : JS_Now(), 0);
+    // mLastModificationDate is an absolute timestamp so we supply a zero context mix-in
   }
 }
 
 void
 MultipartBlobImpl::GetMozFullPathInternal(nsAString& aFilename,
                                           ErrorResult& aRv) const
 {
   if (!mIsFromNsIFile || mBlobImpls.Length() == 0) {

--- a/toolkit/components/resistfingerprinting/nsRFPService.cpp
+++ b/toolkit/components/resistfingerprinting/nsRFPService.cpp
@@ -552,17 +552,17 @@ nsRFPService::ReduceTimePrecisionAsUSecs
 /* static */
 double
 nsRFPService::ReduceTimePrecisionAsUSecsWrapper(double aTime)
 {
   return nsRFPService::ReduceTimePrecisionImpl(
     aTime,
     MicroSeconds,
     TimerResolution(),
-    0,
+    0, /* For absolute timestamps (all the JS engine does), supply zero context mixin */
     TimerPrecisionType::All);
 }
 
 /* static */
 double
 nsRFPService::ReduceTimePrecisionAsMSecs(
   double aTime,
   int64_t aContextMixin,