Is 04/05 April fifth or May fourth? Depends which software you ask. And the software won't tell you it guessed.
This is the test I'd run before any customer list moves from one system to another. You send three fake customers through first. You check where every detail landed. Then you write the format down. After that, the real move is boring. Boring is the goal.
Why dates go wrong
A date like 04/05 is just numbers. Some places write the month first, so it's April 5. Others write the day first, so it's May 4. Your software picks one based on its settings. It doesn't ask you.
ISO, the international standards group, uses this exact problem as its example: 01/05/22 could mean January 5, 2022, or May 1, 2022. Its fix is a format called ISO 8601: year, then month, then day. 2027-03-12 can only mean March 12, 2027.
Here's where it bites. The same file can be read two ways, depending on settings:
- Google Sheets: every spreadsheet has a Locale setting (File, then Settings). Google says changing it changes the sheet's default currency, date and number formatting.
- Excel: Microsoft says that when Excel opens a .csv file, it uses its current default data format settings to read each column. It also removes leading zeros on its own, so customer ID 00412 turns into 412.
So a file can be fine, get opened once "just to look," and come out different. Nobody gets an error message. That's the scary part.
Words you'll see
- Export: pulling your list out of a system as a file.
- Import: loading that file into the new system.
- CSV: a plain text file where commas split the columns. Lots of systems export this way.
- Field: one box on a customer record, like Phone or Appointment date.
- Mapping: telling the new system which column goes into which field.
- Format: how a value is written. 03/12/2027 and 2027-03-12 are the same day in two formats.
Make three fake customers
Three is enough. Each one tests something different. Put TEST in every name so nobody calls them, books them or counts them. Use contact details that can't reach a real person:
- Email: use example.com. IANA, the group that runs internet names and numbers, keeps it for examples, and nobody can register it.
- Phone: your area code plus a number from 555-0100 to 555-0199. NANPA, which runs North American phone numbers, keeps that range as made-up numbers that don't work.
| Fake customer | What it tests | Type this | It should show |
|---|---|---|---|
| 1 · TEST Dateone | Which way the new system reads dates | Appointment 03/12/2027, 9:00 AM | March 12, 2027, 9:00 AM. December 3? It reads day first. |
| 2 · TEST Datetwo | A date that can only mean one thing | Appointment 03/25/2027, 2:30 PM | March 25, 2027, 2:30 PM. An error, a blank or a strange date means it reads day first. |
| 3 · TEST Zoë O'Brien-Smith | The other boxes: customer ID, phone, name, and the safe date format | Customer ID 00412, a phone in the Phone box, appointment 2027-03-12 | ID 00412 with its zeros, the phone in Phone, the name spelled exactly, March 12, 2027. |
TEST CUSTOMER 1: which way?
First name: TEST
Last name: Dateone
Phone: (XXX) 555-0101
Email: test1@example.com
Customer ID: DEMO-0001
Appointment: 03/12/2027 9:00 AM
Should show: March 12, 2027
TEST CUSTOMER 2: one meaning
First name: TEST
Last name: Datetwo
Phone: (XXX) 555-0102
Email: test2@example.com
Customer ID: DEMO-0002
Appointment: 03/25/2027 2:30 PM
Should show: March 25, 2027
TEST CUSTOMER 3: other boxes
First name: TEST Zoë
Last name: O'Brien-Smith
Phone: (XXX) 555-0103
Email: test3@example.com
Customer ID: 00412
Appointment: 2027-03-12 9:00 AM
Should show: March 12, 2027,
ID 00412, zeros kept
XXX = your own area code.Can't add customers to the old system? Add the three as new rows in a copy of the export file instead. Or let the prompt further down build them for your exact columns.
Run the test
Add the three fake customers to the old system
Type them in like any new customer. That tests the whole trip: what the old system sends out, not just what the new one takes in.
Export the same way you'll export for real
Use the same export button and settings you'll use for the real move. Then don't open and save the file in Excel or Sheets on the way. Opening it is how dates get re-read and zeros get dropped. Need to look inside? Open a copy, or open it in a plain text app like Notepad on Windows.
Import it and check the mapping
The new system will ask which column goes where. Slow down here. Look hard at any column called Number, ID or Date. "Number" might be a customer ID, not a phone number. If the import screen lets you pick a date format, pick the one your file really uses. Write down what you picked.
Open each fake customer and check every field
Open the records in the new system. Don't stop at the import summary. "Imported 3 of 3" means three rows went in. It doesn't mean they went in right. Check each field against the "It should show" column above:
- Dates: March 12, not December 3. March 25, not blank.
- Times: 9:00 AM and 2:30 PM, right where you typed them. If a time moved, check the time zone setting in both systems.
- Phone vs customer ID: each in its own box, and ID 00412 still has its zeros.
- Names: Zoë and O'Brien-Smith spelled exactly, with no odd symbols.
Fix the first problem, then run it again
Change one thing: the mapping, the date format on the import screen, or the date setting on the export. Delete the fakes from the new system and send them through again. Repeat until all three come out right.
Write the format down, then clean up
Fill in the format sheet below. Delete the three fake customers from both systems, or tag them TEST so no report counts them. Now do the real move, with the exact same settings.
I've connected a lot of systems. The date box always looks the most innocent. So check it first.
Copy this: the format sheet
One page. Fill it in after the test, keep it with your notes, and hand it to whoever runs the next import. No logins on it.
FORMAT SHEET
From: _____________ (old system)
To: _____________ (new system)
Tested on: ________ By: ________
DATES
Old system exports: ___________
New system reads: ___________
Import setting we picked: ______
Safe format: YYYY-MM-DD
Time zone, old: ______ new: ______
PHONE
Column name in the file: _______
Goes into field: _______________
CUSTOMER ID
Column name in the file: _______
Goes into field: _______________
Zeros kept (00412)? [ ] yes
NAMES
Accents and apostrophes OK? [ ] yes
THE TEST
[ ] Customer 1: March 12, 9:00 AM
[ ] Customer 2: March 25, 2:30 PM
[ ] Customer 3: ID, phone, name OK
[ ] Fake customers deletedDo it with Claude or ChatGPT
Want a hand? Paste this into Claude or ChatGPT. It builds the fake customers for your exact columns, then checks the results with you. Only paste fake data. Never paste real customers into a chat.
I'm moving my customer list from [OLD SYSTEM] to [NEW SYSTEM]. Before the real move, help me run a test with three fake customers. I will only share fake data with you, never real customers.
Here are the column names from my export file (the first line only):
[PASTE THE FIRST LINE OF YOUR EXPORT FILE HERE]
Please:
1. Make three fake test customers that fit these columns. Put TEST in every name. Use emails at example.com. For phones, use my area code and a number from 555-0100 to 555-0199.
- Customer 1: an appointment on 03/12/2027 at 9:00 AM.
- Customer 2: an appointment on 03/25/2027 at 2:30 PM.
- Customer 3: a customer ID that starts with zeros (00412), a name with an accent and an apostrophe (Zoë O'Brien-Smith), and the date written 2027-03-12.
2. Show them two ways: as CSV lines I can paste into a copy of my export file, and as a list I can type in by hand.
3. Go through my columns. Tell me, in plain words, which ones could get mixed up when they move. Look for dates, times, phone numbers vs customer IDs, names, and numbers that start with zero.
4. Then stop and wait. After I import them, I'll paste what the new system shows. Compare it field by field in a table: field, what I typed, what it shows, OK or problem. For each problem, give me the most likely cause in one sentence and the one setting to change.
5. When all three come out right, fill in a one-page format sheet: the date format, the time zone, which column goes into which field, and anything to watch next time.
Use short sentences and plain words. If you need something from me, ask one question at a time.The AI can only compare what you paste. Opening the records in the new system and looking is still your job.
Example (made up)
Coastal Line Roofing is moving its customer list from an old scheduling app to a new customer system. The office manager adds the three fake customers to the old app, exports, and imports. Here's what the new system shows:
| Field | Typed | New system showed | Result |
|---|---|---|---|
| Customer 1 date | 03/12/2027 | Dec 3, 2027 | Problem: it reads day first |
| Customer 2 date | 03/25/2027 | Blank | Problem: same cause |
| Customer 3 date | 2027-03-12 | Mar 12, 2027 | OK |
| Customer 3 phone | (XXX) 555-0103 | 00412 | Problem: "Number" went into Phone |
| Customer 3 ID | 00412 | Empty | Problem: same mix-up |
| Customer 3 name | Zoë O'Brien-Smith | Zoë O'Brien-Smith | OK |
Two fixes. On the import screen, the date setting was day first. She switches it to month first. Then she maps the old app's "Number" column to Customer ID instead of Phone. She deletes the fakes, sends them through again, and all three come out right.
Her format sheet now says: dates go in month first, or as YYYY-MM-DD. "Number" goes into Customer ID. Both systems set to Eastern time. Nobody shows up on the wrong Tuesday.
Mistakes to avoid
Trusting the import summary
Testing with a date like 03/03
Opening the export in a spreadsheet and saving it
Testing with real customers
Not writing it down
That's the test
Not exciting? Correct. Neither is explaining why everyone showed up on the wrong Tuesday.
Checked on Oct 4, 2026 against
Every claim about a third-party tool in this guide (plans, prices, menu paths, commands, limits) was checked against these official pages on Oct 4, 2026. These screens change often: if something looks different, trust the page over this guide.
- ISO: ISO 8601, Date and time formatDates written as numbers can be read different ways (its example: 01/05/22 could be January 5, 2022, or May 1, 2022); ISO 8601 writes dates as YYYY-MM-DD, year then month then day.
- Google Docs Editors Help: Set a spreadsheet's location & calculation settingsFile > Settings > General > Locale and Time zone; changing them changes the spreadsheet's default currency, date and number formatting.
- Microsoft Support: Import or export text (.txt or .csv) filesWhen Excel opens a .csv file, it uses the current default data format settings to interpret each column; Data > From Text/CSV to import with more control.
- Microsoft Support: Keeping leading zeros and large numbersExcel automatically removes leading zeros; columns can be imported as Text to keep them.
- NANPA: 555 line numbers555-0100 through 555-0199 stay reserved as fictitious, non-working numbers.
- IANA: Example domainsexample.com and similar domains are for use as examples in documents (RFC 2606, RFC 6761) and can't be registered.
