diff --git a/src/gtfs/calendar/CalendarFactory.ts b/src/gtfs/calendar/CalendarFactory.ts
index 1e03e34..e974857 100644
--- a/src/gtfs/calendar/CalendarFactory.ts
+++ b/src/gtfs/calendar/CalendarFactory.ts
@@ -1,23 +1,36 @@
 import { Calendar, CalendarDate } from "../GTFS";
-import { toGTFSDate } from "./toGTFSDate";
+import { toGTFSDate, getDateFromGTFSString } from "./gtfsDateUtils";
 
 /**
  * Creates calendars based on a set of calendar dates.
  */
 export class CalendarFactory {
-
   /**
    * Count the number of times a service runs or does not run on each day of the week and then decide whether to
    * create a calendar with that day enabled with exclude days when it is not running, or disabled with include
    * days when it is running.
    */
-  public create(serviceId: string, calendarDates: CalendarDate[]): [Calendar, CalendarDate[]] {
+  public create(
+    serviceId: string,
+    calendarDates: CalendarDate[]
+  ): [Calendar, CalendarDate[]] {
     calendarDates.sort((a, b) => +a.date - +b.date);
 
     const startDate = calendarDates[0];
     const endDate = calendarDates[calendarDates.length - 1];
-    const [daysRunning, daysNotRunning] = this.splitCalendarDates(calendarDates, serviceId, startDate, endDate);
-    const calendar = this.createCalendar(serviceId, startDate, endDate, daysRunning, daysNotRunning);
+    const [daysRunning, daysNotRunning] = this.splitCalendarDates(
+      calendarDates,
+      serviceId,
+      startDate,
+      endDate
+    );
+    const calendar = this.createCalendar(
+      serviceId,
+      startDate,
+      endDate,
+      daysRunning,
+      daysNotRunning
+    );
     const newCalendarDates = this.getCalendarDates(daysRunning, daysNotRunning);
 
     return [calendar, newCalendarDates];
@@ -29,22 +42,28 @@ export class CalendarFactory {
     startDate: CalendarDate,
     endDate: CalendarDate
   ): [CalendarDate[][], CalendarDate[][]] {
-
     const daysNotRunning: CalendarDate[][] = [[], [], [], [], [], [], []];
     const daysRunning: CalendarDate[][] = [[], [], [], [], [], [], []];
     const calendarDateIndex = this.indexCalendarDates(calendarDates);
 
     let i = startDate.date;
-    let date = this.getDateFromGTFSString(i);
+    let date = getDateFromGTFSString(i);
 
     while (i <= endDate.date) {
       const dow = date.getDay();
 
       if (calendarDateIndex[i]) {
-        daysRunning[dow].push({ exception_type: 1, service_id: serviceId, date: i });
-      }
-      else {
-        daysNotRunning[dow].push({ exception_type: 2, service_id: serviceId, date: i });
+        daysRunning[dow].push({
+          exception_type: 1,
+          service_id: serviceId,
+          date: i,
+        });
+      } else {
+        daysNotRunning[dow].push({
+          exception_type: 2,
+          service_id: serviceId,
+          date: i,
+        });
       }
 
       date.setDate(date.getDate() + 1);
@@ -54,7 +73,9 @@ export class CalendarFactory {
     return [daysRunning, daysNotRunning];
   }
 
-  private indexCalendarDates(calendarDates: CalendarDate[]): Record<string, CalendarDate> {
+  private indexCalendarDates(
+    calendarDates: CalendarDate[]
+  ): Record<string, CalendarDate> {
     return calendarDates.reduce((index, calendarDate) => {
       index[calendarDate.date] = calendarDate;
 
@@ -62,10 +83,6 @@ export class CalendarFactory {
     }, {});
   }
 
-  private getDateFromGTFSString(i: string): Date {
-    return new Date(i.substr(0, 4) + "-" + i.substr(4, 2) + "-" + i.substr(6, 2));
-  }
-
   private createCalendar(
     serviceId: string,
     startDate: CalendarDate,
@@ -83,7 +100,7 @@ export class CalendarFactory {
       wednesday: daysRunning[3].length > daysNotRunning[3].length ? 1 : 0,
       thursday: daysRunning[4].length > daysNotRunning[4].length ? 1 : 0,
       friday: daysRunning[5].length > daysNotRunning[5].length ? 1 : 0,
-      saturday: daysRunning[6].length > daysNotRunning[6].length ? 1 : 0
+      saturday: daysRunning[6].length > daysNotRunning[6].length ? 1 : 0,
     };
   }
 
@@ -91,10 +108,14 @@ export class CalendarFactory {
    * For each day of the week check if the service runs more often than not. If it does, return the exclude days as
    * the calendar will have the day set to 1, if not then return the include days.
    */
-  private getCalendarDates(daysRunning: CalendarDate[][], daysNotRunning: CalendarDate[][]): CalendarDate[] {
+  private getCalendarDates(
+    daysRunning: CalendarDate[][],
+    daysNotRunning: CalendarDate[][]
+  ): CalendarDate[] {
     return daysRunning.flatMap((runningDates, i) => {
-      return runningDates.length > daysNotRunning[i].length ? daysNotRunning[i] : runningDates;
+      return runningDates.length > daysNotRunning[i].length
+        ? daysNotRunning[i]
+        : runningDates;
     });
   }
-
 }
diff --git a/src/gtfs/calendar/gftsDateUtils.spec.ts b/src/gtfs/calendar/gftsDateUtils.spec.ts
new file mode 100644
index 0000000..8912071
--- /dev/null
+++ b/src/gtfs/calendar/gftsDateUtils.spec.ts
@@ -0,0 +1,34 @@
+import * as chai from "chai";
+import { toGTFSDate, getDateFromGTFSString } from "./gtfsDateUtils";
+
+describe("toGTFSDate", () => {
+  it("returns a GTFS date string", () => {
+    const date = new Date("2019-06-04T00:00:00");
+    const result = toGTFSDate(date);
+
+    chai.expect(result).to.equal("20190604");
+  });
+});
+
+describe("getDateFromGTFSString", () => {
+  it("returns a JS Date from GTFS string", () => {
+    const result = getDateFromGTFSString("20190604");
+    const dateDiff = result.getTime() - new Date(2019, 5, 4, 0, 0, 0).getTime();
+    chai.expect(dateDiff).to.equal(0);
+  });
+});
+
+describe("GTFS date roundtrip", () => {
+  it("Preserves GTFS dates when converting to/from JS dates", () => {
+    function validateGtfsDateRoundtrip(gtfsDate: string) {
+      const date = getDateFromGTFSString(gtfsDate);
+      const result = toGTFSDate(date);
+      chai.expect(result).to.equal(gtfsDate);
+    }
+
+    validateGtfsDateRoundtrip("20190601");
+    validateGtfsDateRoundtrip("20190604");
+    validateGtfsDateRoundtrip("20191231");
+    validateGtfsDateRoundtrip("20200101");
+  });
+});
diff --git a/src/gtfs/calendar/gtfsDateUtils.ts b/src/gtfs/calendar/gtfsDateUtils.ts
new file mode 100644
index 0000000..39bcc1e
--- /dev/null
+++ b/src/gtfs/calendar/gtfsDateUtils.ts
@@ -0,0 +1,14 @@
+export function toGTFSDate(date: Date): string {
+  return (
+    date.getFullYear() +
+    (date.getMonth() + 1).toString().padStart(2, "0") +
+    date.getDate().toString().padStart(2, "0")
+  );
+}
+
+export function getDateFromGTFSString(i: string): Date {
+  // Zero out the time portion of the date to avoid timezone issues
+  return new Date(
+    i.substr(0, 4) + "-" + i.substr(4, 2) + "-" + i.substr(6, 2) + "T00:00:00"
+  );
+}
diff --git a/src/gtfs/calendar/toGTFSDate.spec.ts b/src/gtfs/calendar/toGTFSDate.spec.ts
deleted file mode 100644
index a606012..0000000
--- a/src/gtfs/calendar/toGTFSDate.spec.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-import * as chai from "chai";
-import { toGTFSDate } from "./toGTFSDate";
-
-describe("toGTFSDate", () => {
-
-  it("returns a GTFS date string", () => {
-    const date = new Date("2019-06-04");
-    const result = toGTFSDate(date);
-
-    chai.expect(result).to.equal("20190604");
-  });
-
-});
diff --git a/src/gtfs/calendar/toGTFSDate.ts b/src/gtfs/calendar/toGTFSDate.ts
deleted file mode 100644
index 615513e..0000000
--- a/src/gtfs/calendar/toGTFSDate.ts
+++ /dev/null
@@ -1,6 +0,0 @@
-
-export function toGTFSDate(date: Date): string {
-  return date.getFullYear()
-    + (date.getMonth() + 1).toString().padStart(2, "0")
-    + date.getDate().toString().padStart(2, "0");
-}
diff --git a/src/index.ts b/src/index.ts
index 64ff34c..70c6c56 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,7 +1,7 @@
 import * as yargs from "yargs";
 import { Arguments } from "yargs";
 import { Container } from "./Container";
-import { toGTFSDate } from "./gtfs/calendar/toGTFSDate";
+import { toGTFSDate } from "./gtfs/calendar/gtfsDateUtils";
 import { RouteType } from "./gtfs/GTFS";
 
 const args = yargs.argv as Arguments<{