"What date is 3 weeks from now?" seems like an easy question. But ask an LLM on January 30th and you might get February 20th (wrong), February 21st (maybe), or even March 1st (definitely wrong). Here's why AI fails at dates and how MCP tools fix it.
Why LLMs Fail at Date Math
Large Language Models are trained on text, not calendars. When they encounter date questions, they're essentially pattern-matching against examples they've seen, not computing actual dates.
Common Date Calculation Failures
| Problem | What LLMs Often Miss |
|---|---|
| Month lengths | February has 28/29 days, months vary from 28-31 |
| Leap years | 2024 is a leap year, 2100 isn't (century rule) |
| Weekends | Business day calculations skip Sat/Sun |
| DST transitions | Clocks shift, affecting time calculations |
| Timezone offsets | Not all zones are whole-hour offsets |
Example of LLM Date Failure
Without MCP Tools
User: "It's January 30, 2025. When is 30 days from now?"
LLM: "30 days from January 30, 2025 is February 29, 2025."
Reality: February 29, 2025 doesn't exist!
2025 is not a leap year.
Correct answer: March 1, 2025
The Core Problem: LLMs don't have a running calendar. They don't know "today's date" unless you tell them, and even then, they approximate rather than calculate.
TinyFn DateTime Endpoints
TinyFn provides a comprehensive set of datetime tools via MCP. These tools actually compute dates using proper calendar libraries.
Available DateTime Tools
| Tool | Purpose |
|---|---|
datetime/now |
Get current date/time in any timezone |
datetime/add |
Add years, months, days, hours to a date |
datetime/difference |
Calculate time between two dates |
datetime/add-business-days |
Add days excluding weekends |
datetime/format |
Format dates in any pattern |
datetime/parse |
Parse dates from various formats |
datetime/start-of |
Get start of day/week/month/year |
datetime/end-of |
Get end of day/week/month/year |
Basic Date Addition
API Request - Add 30 Days
GET https://api.tinyfn.io/v1/datetime/add?date=2025-01-30&days=30
Headers: X-API-Key: your-api-key
Response
{
"original": "2025-01-30T00:00:00.000Z",
"result": "2025-03-01T00:00:00.000Z",
"added": {
"days": 30
},
"formatted": "Saturday, March 1, 2025"
}Timezone Handling
Timezone calculations are notoriously tricky. TinyFn uses the IANA timezone database and handles all the edge cases automatically.
Convert Between Timezones
Timezone Conversion
GET https://api.tinyfn.io/v1/time/convert
?datetime=2025-01-30T15:00:00
&from=America/New_York
&to=Asia/Tokyo
Response
{
"from": {
"datetime": "2025-01-30T15:00:00-05:00",
"timezone": "America/New_York",
"offset": "-05:00"
},
"to": {
"datetime": "2025-01-31T05:00:00+09:00",
"timezone": "Asia/Tokyo",
"offset": "+09:00"
},
"difference_hours": 14
}DST-Aware Calculations
TinyFn automatically handles Daylight Saving Time transitions:
DST Handling
// Adding 1 day across DST transition
GET https://api.tinyfn.io/v1/datetime/add
?date=2025-03-08T12:00:00
&timezone=America/New_York
&days=1
// Returns March 9th, 2025 at 12:00 PM EST (not 11 AM or 1 PM)
// Correctly handles the "spring forward" transitionBusiness Days and Holidays
One of the most common date calculation needs is business day math. "When will this ship if we process in 5 business days?" requires skipping weekends and potentially holidays.
Add Business Days
Business Days Calculation
GET https://api.tinyfn.io/v1/datetime/add-business-days
?date=2025-01-30
&days=5
Response
{
"start_date": "2025-01-30",
"business_days_added": 5,
"result": "2025-02-06",
"calendar_days": 7,
"skipped_weekend_days": 2
}Include Holiday Exclusions
Business Days with Holidays
GET https://api.tinyfn.io/v1/datetime/add-business-days
?date=2025-12-20
&days=5
&holidays=2025-12-25,2025-12-26,2026-01-01
Response
{
"start_date": "2025-12-20",
"business_days_added": 5,
"result": "2025-12-31",
"calendar_days": 11,
"skipped_weekend_days": 2,
"skipped_holidays": ["2025-12-25", "2025-12-26"]
}Calculate Business Days Between Dates
Business Day Count
GET https://api.tinyfn.io/v1/datetime/business-day-difference
?start=2025-01-01
&end=2025-01-31
Response
{
"start": "2025-01-01",
"end": "2025-01-31",
"business_days": 22,
"calendar_days": 30,
"weekends": 8
}Real-World Examples
Shipping Estimate Calculation
E-commerce Shipping
// Customer orders on Friday afternoon
// Warehouse processes next business day
// Then 3-5 business days for shipping
// Step 1: Get next business day (processing day)
datetime/add-business-days?date=2025-01-31&days=1
// Result: 2025-02-03 (Monday)
// Step 2: Add shipping window
datetime/add-business-days?date=2025-02-03&days=3
// Result: 2025-02-06 (Thursday) - earliest arrival
datetime/add-business-days?date=2025-02-03&days=5
// Result: 2025-02-10 (Monday) - latest arrival
// Response: "Arrives Feb 6-10"Subscription Renewal Date
SaaS Billing
// User signed up January 31
// Monthly subscription - when's next billing?
datetime/add?date=2025-01-31&months=1
// Result: 2025-02-28 (last day of Feb, since Feb 31 doesn't exist)
// The tool correctly handles month-end edge cases!International Meeting Scheduler
Multi-timezone Meeting
// Find a meeting time that works for NYC, London, and Tokyo
// Propose 9 AM EST
time/convert?datetime=2025-01-30T09:00&from=America/New_York&to=Europe/London
// Result: 2:00 PM GMT
time/convert?datetime=2025-01-30T09:00&from=America/New_York&to=Asia/Tokyo
// Result: 11:00 PM JST
// That's too late for Tokyo. Try 7 AM EST instead:
time/convert?datetime=2025-01-30T07:00&from=America/New_York&to=Asia/Tokyo
// Result: 9:00 PM JST - Better!Age Calculator
Calculate Exact Age
datetime/difference?start=1990-06-15&end=2025-01-30
Response
{
"difference": {
"years": 34,
"months": 7,
"days": 15
},
"total_days": 12648,
"human_readable": "34 years, 7 months, 15 days"
}Integration Guide
MCP Configuration
mcp.json
{
"mcpServers": {
"tinyfn-datetime": {
"url": "https://api.tinyfn.io/mcp/datetime/",
"headers": {
"X-API-Key": "your-api-key"
}
}
}
}Python Integration
import requests
class DateTimeTools:
def __init__(self, api_key):
self.api_key = api_key
self.base_url = "https://api.tinyfn.io/v1"
def add_business_days(self, date, days, holidays=None):
params = {"date": date, "days": days}
if holidays:
params["holidays"] = ",".join(holidays)
response = requests.get(
f"{self.base_url}/datetime/add-business-days",
params=params,
headers={"X-API-Key": self.api_key}
)
return response.json()
def convert_timezone(self, datetime_str, from_tz, to_tz):
response = requests.get(
f"{self.base_url}/time/convert",
params={"datetime": datetime_str, "from": from_tz, "to": to_tz},
headers={"X-API-Key": self.api_key}
)
return response.json()
# Usage
tools = DateTimeTools("your-api-key")
shipping_date = tools.add_business_days("2025-01-30", 5)
print(shipping_date["result"]) # 2025-02-06JavaScript Integration
class DateTimeTools {
constructor(apiKey) {
this.apiKey = apiKey;
this.baseUrl = 'https://api.tinyfn.io/v1';
}
async addBusinessDays(date, days, holidays = []) {
const params = new URLSearchParams({ date, days });
if (holidays.length) {
params.set('holidays', holidays.join(','));
}
const response = await fetch(
`${this.baseUrl}/datetime/add-business-days?${params}`,
{ headers: { 'X-API-Key': this.apiKey } }
);
return response.json();
}
async convertTimezone(datetime, fromTz, toTz) {
const params = new URLSearchParams({ datetime, from: fromTz, to: toTz });
const response = await fetch(
`${this.baseUrl}/time/convert?${params}`,
{ headers: { 'X-API-Key': this.apiKey } }
);
return response.json();
}
}
// Usage
const tools = new DateTimeTools('your-api-key');
const result = await tools.addBusinessDays('2025-01-30', 5);
console.log(result.result); // 2025-02-06Never Get Date Calculations Wrong Again
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