docs: add guide for calculating effective entry date

docs/calculating-effective-entry-date.md

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+---
+layout: page
+title: Calculating Effective Entry Date 
+hide_hero: true
+show_sidebar: false
+menubar: docs-menu
+---
+
+# Effective Entry Date Field Constraints
+
+## 1. Transmission Windows for Same-Day ACH
+
+There are three transmission deadlines:
+
+* **1st Cutoff**: 10:30 a.m. ET
+* **2nd Cutoff**: 2:45 p.m. ET
+* **3rd Cutoff**: 4:45 p.m. ET
+
+Files must be submitted before the last cutoff time to be eligible for same-day processing.
+
+If a file is submitted after the last cutoff, entries that list the current banking day as their Effective Entry Date will be settled on the next banking day.
+
+## 2. Same-Day ACH Transaction Limit
+
+* The per-entry limit for same-day ACH is **$1,000,000**
+* Entries exceeding this amount are not eligible for same-day settlement
+* In a batch containing entries over the limit, those entries will have their Effective Entry Date adjusted to the next banking day and remaining items will still be eligible for same-day settlement
+
+## 3. Credit Entries
+
+For credit entries, the Effective Entry Date must be either:
+* The **same banking day** as the processing date for **Same Day Entries**, or
+* **One or two banking days** following the processing date for other entries
+
+## 4. Debit Entries
+
+For debit entries, the Effective Entry Date must be either:
+* The **same banking day** as the processing date for **Same Day Entries**, or
+* **One banking day** following the processing date for other entries
+
+**Note:** Receiving banks may place holds on ACH debits per their funds availability policies, separate from these settlement times.
+
+## 5. Effective Entry Date - Edge Cases
+
+### ENR Entries
+* For Automated Enrollment Entries (ENR), the Effective Entry Date field must be **space-filled**
+* ENR entries cannot be Same Day Entries
+
+### Notifications of Change
+* The ACH Operator does **not edit** the Effective Entry Date field for Notifications of Change
+
+### Return Entries
+* The ACH Operator may verify that the Effective Entry Date is properly formatted and may replace the field's existing content with the current processing date if the Effective Entry Date is invalid
+
+# Calculating Effective Entry Dates Using moov-io/base Functions
+
+## Overview
+
+The moov-io/base package provides functionality for calculating valid Effective Entry Dates when creating ACH batch headers, ensuring compliance with NACHA rules and Federal Reserve processing schedules.
+
+## Creating Batch Headers
+
+The NewBatchHeader function creates a BatchHeader object that requires several key fields to be populated according to NACHA rules:
+
+```go
+bh := NewBatchHeader()
+bh.ServiceClassCode = "220"                  // 220 for credits only
+bh.StandardEntryClassCode = PPD              // PPD, CCD, WEB, etc.
+bh.CompanyName = "ACME Corporation"          // Your company name
+bh.CompanyIdentification = "121042882"       // Your company ID
+bh.CompanyEntryDescription = "PAYROLL"       // Transaction description
+bh.ODFIIdentification = "12104288"           // Your bank's routing number
+
+bh.EffectiveEntryDate = "240102"             // YYMMDD format
+```
+
+## Banking Day Functions
+
+The moov-io/base Time package provides these essential functions for determining valid banking days:
+
+* `AddBankingTime(hours, minutes, seconds int)`: Increments time by specified duration while respecting banking hours (9am-5pm ET)
+* `AddBankingDay(d int)`: Adds the specified number of valid banking days to a date
+* `IsBankingDay()`: Validates if a date is a banking day by checking both holidays and weekends
+* `IsBusinessDay()`: Determines if a date is a standard business day
+* `IsHoliday()`: Checks if a date falls on a Federal Reserve holiday
+* `IsWeekend()`: Verifies if a date falls on Saturday or Sunday
+
+## Transmission Windows
+
+The package handles Same-Day ACH transmission windows with time-based validations. Here's how to implement the cutoff time checks:
+
+```go
+now := base.Now()
+// Create cutoff times for each window
+firstCutoff := now.AddBankingTime(10, 30, 0)    // 10:30 AM ET
+secondCutoff := now.AddBankingTime(14, 45, 0)   // 2:45 PM ET
+thirdCutoff := now.AddBankingTime(16, 45, 0)    // 4:45 PM ET
+
+if now.Before(firstCutoff.Time) {
+   // In first window
+   bh.EffectiveEntryDate = now.Format("060102")
+} else if now.Before(secondCutoff.Time) {
+   // First window closed, in second window
+   bh.EffectiveEntryDate = now.Format("060102")
+} else if now.Before(thirdCutoff.Time) {
+   // Second window closed, in third window
+   bh.EffectiveEntryDate = now.Format("060102")
+} else {
+   // All windows closed for today
+   now = now.AddBankingDay(1)
+   bh.EffectiveEntryDate = now.Format("060102")
+}
+```
+
+## Implementation Example
+
+Here's a complete example showing how to calculate an Effective Entry Date based on various conditions:
+
+```go
+func calculateEffectiveEntryDate(amount int, isCredit bool) string {
+   now := base.Now()
+
+   // On weekends and holidays, entries must be processed on future banking days
+   if !now.IsBankingDay() {
+       if amount <= 1000000 {
+           // Same day entries settle on the next available banking day
+           // Example: Saturday file -> Monday settlement
+           now = now.AddBankingDay(1)
+           return now.Format("060102")
+       } else {
+           // Non same-day entries must settle beyond the next banking day
+           // to prevent inadvertent same-day settlement
+           // Example: Saturday file -> Tuesday settlement (skipping Monday)
+           now = now.AddBankingDay(1) // Move to next banking day (Monday)
+           now = now.AddBankingDay(1) // Move one more day (Tuesday) to maintain non same-day settlement
+           return now.Format("060102")
+       }
+   }
+
+   // The last cutoff for same-day ACH is 4:45 PM ET
+   thirdCutoff := now.AddBankingTime(16, 45, 0)
+   if now.After(thirdCutoff.Time) {
+       // After the cutoff, entries follow the same rules as non-banking days
+       if amount <= 1000000 {
+           // Same day entries settle next banking day
+           now = now.AddBankingDay(1)
+       } else {
+           // Non same-day entries must settle two banking days out
+           now = now.AddBankingDay(2)
+       }
+       return now.Format("060102")
+   }
+
+   // Before the cutoff on a banking day, same-day settlement is available
+   if amount <= 1000000 {
+       // Entries within same-day limits can settle today
+       return now.Format("060102")
+   }
+
+   // Non same-day entries must settle at least one banking day after creation
+   now = now.AddBankingDay(1)
+   return now.Format("060102")
+}