diff --git a/Sources/LoopAlgorithm/SampleValue.swift b/Sources/LoopAlgorithm/SampleValue.swift
index fb978b2..28929e2 100644
--- a/Sources/LoopAlgorithm/SampleValue.swift
+++ b/Sources/LoopAlgorithm/SampleValue.swift
@@ -111,6 +111,13 @@ public extension Sequence where Element: TimelineValue {
 public extension RandomAccessCollection where Element: TimelineValue, Index == Int {
     func filterDateRange(_ startDate: Date?, _ endDate: Date?) -> [Element] {
         guard !isEmpty else { return [] }
+        // This binary-search filter is only correct when the elements are sorted
+        // ascending by startDate. Catch contract violations in debug builds; the
+        // check is compiled out of release builds, so there is no runtime cost.
+        assert(
+            zip(self, dropFirst()).allSatisfy { $0.startDate <= $1.startDate },
+            "filterDateRange requires elements sorted ascending by startDate"
+        )
         // Lower bound: first index where element.endDate >= startDate
         var lo = startIndex
         if let startDate {
diff --git a/Tests/LoopAlgorithmTests/Fixtures/effect_from_history_output.json b/Tests/LoopAlgorithmTests/Fixtures/effect_from_history_output.json
index 26aabee..9f5feaa 100644
--- a/Tests/LoopAlgorithmTests/Fixtures/effect_from_history_output.json
+++ b/Tests/LoopAlgorithmTests/Fixtures/effect_from_history_output.json
@@ -155,489 +155,488 @@
     "unit" : "mg/dL"
   },
   {
-    "amount" : -52.264027741079879,
+    "amount" : -52.2640280422766,
     "date" : "2015-10-15T20:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -61.76646699369725,
+    "amount" : -61.7677478059354,
     "date" : "2015-10-15T20:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -71.680918819657265,
+    "amount" : -71.687637842502099,
     "date" : "2015-10-15T20:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -81.900465930344126,
+    "amount" : -81.919234933234577,
     "date" : "2015-10-15T20:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -92.328165568362309,
+    "amount" : -92.367908821205617,
     "date" : "2015-10-15T21:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -102.87748849853307,
+    "amount" : -102.94913050298391,
     "date" : "2015-10-15T21:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -113.47146208796229,
+    "amount" : -113.58764235562131,
     "date" : "2015-10-15T21:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -124.06101315660501,
+    "amount" : -124.23414028175458,
     "date" : "2015-10-15T21:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -134.62009905018957,
+    "amount" : -134.86086625464586,
     "date" : "2015-10-15T21:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -145.1217841778539,
+    "amount" : -145.43932554945283,
     "date" : "2015-10-15T21:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -155.53243783066668,
+    "amount" : -155.93447255166663,
     "date" : "2015-10-15T21:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -165.81576158842529,
+    "amount" : -166.30872699287778,
     "date" : "2015-10-15T21:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -175.94014779129205,
+    "amount" : -176.52932203535278,
     "date" : "2015-10-15T21:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -185.87826522474734,
+    "amount" : -186.56788030227949,
     "date" : "2015-10-15T21:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -195.96262752949394,
+    "amount" : -196.75597331694593,
     "date" : "2015-10-15T21:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -206.52914016717139,
+    "amount" : -207.42866013639221,
     "date" : "2015-10-15T21:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -217.47340056280586,
+    "amount" : -218.48077975800794,
     "date" : "2015-10-15T22:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -228.70047046491561,
+    "amount" : -229.81671630023288,
     "date" : "2015-10-15T22:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -240.12529726332727,
+    "amount" : -241.35081366017906,
     "date" : "2015-10-15T22:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -251.690438990204,
+    "amount" : -253.02509433881008,
     "date" : "2015-10-15T22:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -263.36153925216848,
+    "amount" : -264.80472864892096,
     "date" : "2015-10-15T22:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -275.10715080098271,
+    "amount" : -276.65785298282833,
     "date" : "2015-10-15T22:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -286.89908382548953,
+    "amount" : -288.55591319388481,
     "date" : "2015-10-15T22:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -298.71214261916953,
+    "amount" : -300.47339671941916,
     "date" : "2015-10-15T22:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -310.52328534383491,
+    "amount" : -312.38698814059592,
     "date" : "2015-10-15T22:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -322.29421713089812,
+    "amount" : -324.25815839722361,
     "date" : "2015-10-15T22:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -333.97464519098088,
+    "amount" : -336.03641630638566,
     "date" : "2015-10-15T22:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -345.52048542110253,
+    "amount" : -347.67751185366495,
     "date" : "2015-10-15T22:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -356.8933206733189,
+    "amount" : -359.14289140170098,
     "date" : "2015-10-15T23:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -368.05989966830873,
+    "amount" : -370.39919378169293,
     "date" : "2015-10-15T23:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -378.99167375942488,
+    "amount" : -381.41778445665733,
     "date" : "2015-10-15T23:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -389.66436893407285,
+    "amount" : -392.17432512769443,
     "date" : "2015-10-15T23:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -400.05759060934326,
+    "amount" : -402.64837632560784,
     "date" : "2015-10-15T23:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -410.15445893826853,
+    "amount" : -412.82303069063533,
     "date" : "2015-10-15T23:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -419.94127249255814,
+    "amount" : -422.68457479343311,
     "date" : "2015-10-15T23:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -429.40719832776898,
+    "amount" : -432.22217749140992,
     "date" : "2015-10-15T23:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -438.54398656819569,
+    "amount" : -441.42760294662321,
     "date" : "2015-10-15T23:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -447.34570777179891,
+    "amount" : -450.2949465552303,
     "date" : "2015-10-15T23:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -455.80851145079703,
+    "amount" : -458.82039215448776,
     "date" : "2015-10-15T23:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -463.93040423153343,
+    "amount" : -467.00198898193264,
     "date" : "2015-10-15T23:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -471.71104623839892,
+    "amount" : -474.83944696315064,
     "date" : "2015-10-16T00:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -479.15156438132186,
+    "amount" : -482.33394899984762,
     "date" : "2015-10-16T00:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -486.2543813150387,
+    "amount" : -489.48797901916589,
     "date" : "2015-10-16T00:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -493.023058921406,
+    "amount" : -496.3051646287397,
     "date" : "2015-10-16T00:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -499.46215486351923,
+    "amount" : -502.79013291995432,
     "date" : "2015-10-16T00:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -505.5770310131204,
+    "amount" : -508.94831821502453,
     "date" : "2015-10-16T00:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -511.37357039402116,
+    "amount" : -514.78567839516109,
     "date" : "2015-10-16T00:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -516.85813478586385,
+    "amount" : -520.30865194907335,
     "date" : "2015-10-16T00:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -522.03765726190443,
+    "amount" : -525.52425001077142,
     "date" : "2015-10-16T00:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -526.91960305812609,
+    "amount" : -530.44001677959045,
     "date" : "2015-10-16T00:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -531.51186924154661,
+    "amount" : -535.0639287862225,
     "date" : "2015-10-16T00:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -535.82267812482121,
+    "amount" : -539.40428794807474,
     "date" : "2015-10-16T00:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -539.86069160301474,
+    "amount" : -543.46983558631189,
     "date" : "2015-10-16T01:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -543.63492920431247,
+    "amount" : -547.26967019309222,
     "date" : "2015-10-16T01:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -547.15451907267538,
+    "amount" : -550.81299816397518,
     "date" : "2015-10-16T01:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -550.42846838462219,
+    "amount" : -554.1089039948821,
     "date" : "2015-10-16T01:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -553.46562056285904,
+    "amount" : -557.16630730372844,
     "date" : "2015-10-16T01:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -556.27482496187201,
+    "amount" : -559.994132349437,
     "date" : "2015-10-16T01:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -558.86493785268158,
+    "amount" : -562.60130887330024,
     "date" : "2015-10-16T01:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -561.24477785349723,
+    "amount" : -564.99672740737037,
     "date" : "2015-10-16T01:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -563.43359411530798,
+    "amount" : -567.19970735700224,
     "date" : "2015-10-16T01:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -565.45900786932339,
+    "amount" : -569.23793846270314,
     "date" : "2015-10-16T01:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -567.32945007449439,
+    "amount" : -571.11991837788901,
     "date" : "2015-10-16T01:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -569.05270789672886,
+    "amount" : -572.85349909266051,
     "date" : "2015-10-16T01:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -570.6363878436191,
+    "amount" : -574.44635002602649,
     "date" : "2015-10-16T02:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -572.0877702437632,
+    "amount" : -575.9058124743799,
     "date" : "2015-10-16T02:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -573.41396516542841,
+    "amount" : -577.23905550979885,
     "date" : "2015-10-16T02:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -574.6219981579759,
+    "amount" : -578.45316171050558,
     "date" : "2015-10-16T02:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -575.7275007450005,
+    "amount" : -579.56381765143828,
     "date" : "2015-10-16T02:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -576.74580130146126,
+    "amount" : -580.5864047863015,
     "date" : "2015-10-16T02:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -577.68106518094305,
+    "amount" : -581.52513958638849,
     "date" : "2015-10-16T02:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -578.53713343337517,
+    "amount" : -582.38391227573231,
     "date" : "2015-10-16T02:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -579.31768623633491,
+    "amount" : -583.16645029377958,
     "date" : "2015-10-16T02:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -580.02630527022711,
+    "amount" : -583.87641862524356,
     "date" : "2015-10-16T02:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -580.66622392121678,
+    "amount" : -584.51718221441229,
     "date" : "2015-10-16T02:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -581.24043672229402,
+    "amount" : -585.09185723978885,
     "date" : "2015-10-16T02:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -581.7518346710234,
+    "amount" : -585.60345407259592,
     "date" : "2015-10-16T03:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -582.20324063049941,
+    "amount" : -586.05490729716598,
     "date" : "2015-10-16T03:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -582.59740660912655,
+    "amount" : -586.44907327579324,
     "date" : "2015-10-16T03:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -582.93757898138779,
+    "amount" : -586.78924564805448,
     "date" : "2015-10-16T03:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -583.22741489495911,
+    "amount" : -587.0790815616258,
     "date" : "2015-10-16T03:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -583.47030192860075,
+    "amount" : -587.32196859526744,
     "date" : "2015-10-16T03:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -583.66919134980412,
+    "amount" : -587.52085801647081,
     "date" : "2015-10-16T03:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -583.8267089590006,
+    "amount" : -587.67837562566717,
     "date" : "2015-10-16T03:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -583.94537949189805,
+    "amount" : -587.79704615856474,
     "date" : "2015-10-16T03:40:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.02762789860844,
+    "amount" : -587.87929456527513,
     "date" : "2015-10-16T03:45:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.08632593727748,
+    "amount" : -587.93799260394417,
     "date" : "2015-10-16T03:50:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.13502294212412,
+    "amount" : -587.98668960879081,
     "date" : "2015-10-16T03:55:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.17434487516277,
+    "amount" : -588.02601154182946,
     "date" : "2015-10-16T04:00:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.20485800454787,
+    "amount" : -588.05652467121456,
     "date" : "2015-10-16T04:05:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.22710644921244,
+    "amount" : -588.07877311587902,
     "date" : "2015-10-16T04:10:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.24216197536089,
+    "amount" : -588.09382864202757,
     "date" : "2015-10-16T04:15:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.25155129938207,
+    "amount" : -588.10321796604876,
     "date" : "2015-10-16T04:20:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.2566906636182,
+    "amount" : -588.10835733028489,
     "date" : "2015-10-16T04:25:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.2589075785753,
+    "amount" : -588.11057424524188,
     "date" : "2015-10-16T04:30:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.2594444444444,
+    "amount" : -588.11111111111109,
     "date" : "2015-10-16T04:35:00",
     "unit" : "mg/dL"
   },
   {
-    "amount" : -584.2594444444444,
+    "amount" : -588.11111111111109,
     "date" : "2015-10-16T04:40:00",
     "unit" : "mg/dL"
   }
-]
-
+]
\ No newline at end of file
diff --git a/Tests/LoopAlgorithmTests/InsulinMathTests.swift b/Tests/LoopAlgorithmTests/InsulinMathTests.swift
index 04b74c2..bc5b47a 100644
--- a/Tests/LoopAlgorithmTests/InsulinMathTests.swift
+++ b/Tests/LoopAlgorithmTests/InsulinMathTests.swift
@@ -237,17 +237,10 @@ class InsulinMathTests: XCTestCase {
             )
         ]
 
+        // A well-formed basal schedule: sorted ascending, contiguous, and non-overlapping.
         let basal = [
             AbsoluteScheduleValue(
                 startDate: startDate,
-                endDate: dateFormatter.date(from: "2015-10-15T20:30:00")!,
-                value: 1.0),
-            AbsoluteScheduleValue(
-                startDate: dateFormatter.date(from: "2015-10-15T20:30:00")!,
-                endDate: dateFormatter.date(from: "2015-10-15T21:00:00")!,
-                value: 0.8),
-            AbsoluteScheduleValue(
-                startDate: dateFormatter.date(from: "2015-10-15T21:00:00")!,
                 endDate: dateFormatter.date(from: "2015-10-15T18:30:00")!,
                 value: 1.0),
             AbsoluteScheduleValue(
@@ -256,6 +249,14 @@ class InsulinMathTests: XCTestCase {
                 value: 0.8),
             AbsoluteScheduleValue(
                 startDate: dateFormatter.date(from: "2015-10-15T19:00:00")!,
+                endDate: dateFormatter.date(from: "2015-10-15T20:30:00")!,
+                value: 1.0),
+            AbsoluteScheduleValue(
+                startDate: dateFormatter.date(from: "2015-10-15T20:30:00")!,
+                endDate: dateFormatter.date(from: "2015-10-15T21:00:00")!,
+                value: 0.8),
+            AbsoluteScheduleValue(
+                startDate: dateFormatter.date(from: "2015-10-15T21:00:00")!,
                 endDate: endDate,
                 value: 1.0),
         ]