diff --git a/ts/lib/BUILD.bazel b/ts/lib/BUILD.bazel
index e3f565b59..627396e19 100644
--- a/ts/lib/BUILD.bazel
+++ b/ts/lib/BUILD.bazel
@@ -2,6 +2,7 @@ load("@npm//@bazel/typescript:index.bzl", "ts_library")
 load("//ts:prettier.bzl", "prettier_test")
 load("//ts:eslint.bzl", "eslint_test")
 load("//ts:protobuf.bzl", "protobufjs_library")
+load("@npm//jest-cli:index.bzl", "jest_test")
 
 # Protobuf
 #############
@@ -44,7 +45,10 @@ genrule(
 
 ts_library(
     name = "lib",
-    srcs = glob(["**/*.ts"]) + [":i18n.ts"],
+    srcs = glob(
+        ["**/*.ts"],
+        exclude = ["*.test.ts"],
+    ) + [":i18n.ts"],
     data = [
         "backend_proto",
     ],
@@ -72,3 +76,31 @@ eslint_test(
     name = "eslint",
     srcs = glob(["*.ts"]),
 )
+
+ts_library(
+    name = "test_lib",
+    srcs = glob(["*.test.ts"]),
+    tsconfig = "//ts:tsconfig.json",
+    deps = [
+        ":lib",
+        "@npm//@types/jest",
+    ],
+)
+
+jest_test(
+    name = "test",
+    args = [
+        "--no-cache",
+        "--no-watchman",
+        "--ci",
+        "--colors",
+        "--config",
+        "$(location //ts:jest.config.js)",
+    ],
+    data = [
+        ":test_lib",
+        "//ts:jest.config.js",
+        "//ts:package.json",
+        "@npm//protobufjs",
+    ],
+)
diff --git a/ts/lib/time.test.ts b/ts/lib/time.test.ts
new file mode 100644
index 000000000..64a86b9de
--- /dev/null
+++ b/ts/lib/time.test.ts
@@ -0,0 +1,25 @@
+import { naturalUnit, naturalWholeUnit, TimespanUnit } from "./time";
+
+test("natural unit", () => {
+    expect(naturalUnit(5)).toBe(TimespanUnit.Seconds);
+    expect(naturalUnit(59)).toBe(TimespanUnit.Seconds);
+    expect(naturalUnit(60)).toBe(TimespanUnit.Minutes);
+    expect(naturalUnit(60 * 60 - 1)).toBe(TimespanUnit.Minutes);
+    expect(naturalUnit(60 * 60)).toBe(TimespanUnit.Hours);
+    expect(naturalUnit(60 * 60 * 24)).toBe(TimespanUnit.Days);
+    expect(naturalUnit(60 * 60 * 24 * 30)).toBe(TimespanUnit.Months);
+});
+
+test("natural whole unit", () => {
+    expect(naturalWholeUnit(5)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(59)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(60)).toBe(TimespanUnit.Minutes);
+    expect(naturalWholeUnit(61)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(90)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(60 * 60 - 1)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(60 * 60 + 1)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(60 * 60)).toBe(TimespanUnit.Hours);
+    expect(naturalWholeUnit(24 * 60 * 60 - 1)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(24 * 60 * 60 + 1)).toBe(TimespanUnit.Seconds);
+    expect(naturalWholeUnit(24 * 60 * 60)).toBe(TimespanUnit.Days);
+});
diff --git a/ts/lib/time.ts b/ts/lib/time.ts
index cd455c4fe..c59e4fd8a 100644
--- a/ts/lib/time.ts
+++ b/ts/lib/time.ts
@@ -10,7 +10,7 @@ export const DAY = 24.0 * HOUR;
 export const MONTH = 30.0 * DAY;
 export const YEAR = 12.0 * MONTH;
 
-enum TimespanUnit {
+export enum TimespanUnit {
     Seconds,
     Minutes,
     Hours,
@@ -53,23 +53,41 @@ export function naturalUnit(secs: number): TimespanUnit {
     }
 }
 
-export function unitAmount(unit: TimespanUnit, secs: number): number {
+/// Number of seconds in a given unit.
+export function unitSeconds(unit: TimespanUnit): number {
     switch (unit) {
         case TimespanUnit.Seconds:
-            return secs;
+            return SECOND;
         case TimespanUnit.Minutes:
-            return secs / MINUTE;
+            return MINUTE;
         case TimespanUnit.Hours:
-            return secs / HOUR;
+            return HOUR;
         case TimespanUnit.Days:
-            return secs / DAY;
+            return DAY;
         case TimespanUnit.Months:
-            return secs / MONTH;
+            return MONTH;
         case TimespanUnit.Years:
-            return secs / YEAR;
+            return YEAR;
     }
 }
 
+export function unitAmount(unit: TimespanUnit, secs: number): number {
+    return secs / unitSeconds(unit);
+}
+
+/// Largest unit provided seconds can be divided by without a remainder.
+export function naturalWholeUnit(secs: number): TimespanUnit {
+    let unit = naturalUnit(secs);
+    while (unit != TimespanUnit.Seconds) {
+        const amount = Math.round(unitAmount(unit, secs));
+        if (Math.abs(secs - amount * unitSeconds(unit)) < Number.EPSILON) {
+            return unit;
+        }
+        unit -= 1;
+    }
+    return unit;
+}
+
 export function studiedToday(cards: number, secs: number): string {
     const unit = naturalUnit(secs);
     const amount = unitAmount(unit, secs);