diff --git a/config.go b/config.go
index 9d79a1c..1744dfa 100644
--- a/config.go
+++ b/config.go
@@ -4,19 +4,21 @@ import (
 	"fmt"
 	"os"
 
+	"github.com/google/uuid"
 	"gopkg.in/yaml.v3"
 )
 
 type config struct {
-	YnabToken       string   `yaml:"ynabToken"`
-	BudgetId        string   `yaml:"budgetId"`
-	SplitAccountIds []string `yaml:"splitAccountIds"`
+	YnabToken       string      `yaml:"ynabToken"`
+	BudgetId        uuid.UUID   `yaml:"budgetId"`
+	SplitCategoryId uuid.UUID   `yaml:"splitCategoryId"`
+	SplitAccountIds []uuid.UUID `yaml:"splitAccountIds"`
 }
 
 func LoadConfig() (*config, error) {
 	f, err := os.Open("config.yml")
 	if err != nil {
-		return nil, fmt.Errorf("Error opening config file. Did you create a config.yml?\n\t%w\n", err)
+		return nil, fmt.Errorf("error opening config file. Did you create a config.yml?\n\t%w\n", err)
 	}
 	defer f.Close()
 
@@ -25,17 +27,31 @@ func LoadConfig() (*config, error) {
 	err = decoder.Decode(&cfg)
 	if err != nil {
 		return nil, fmt.Errorf(
-			"Error decoding config file. Make sure it has the correct format. See config.yml.example for an example\n\t%w\n",
+			"error decoding config file. Make sure it has the correct format. See config.yml.example for an example\n\t%w\n",
 			err,
 		)
 	}
 
+	missingFields := make([]string, 0)
+	fmt.Println(cfg.SplitCategoryId)
 	if len(cfg.YnabToken) == 0 {
-		return nil, fmt.Errorf("Error: config file is missing ynabToken")
+		missingFields = append(missingFields, "ynabToken")
+	}
+	if cfg.BudgetId == uuid.Nil {
+		missingFields = append(missingFields, "budgetId")
+	}
+	if cfg.SplitCategoryId == uuid.Nil {
+		missingFields = append(missingFields, "splitCategoryId")
+	}
+
+	if len(missingFields) > 0 {
+		return nil, fmt.Errorf("missing required fields in config file: %v", missingFields)
 	}
 
-	if len(cfg.BudgetId) == 0 {
-		return nil, fmt.Errorf("Error: config file is missing budgetId")
+	for _, id := range cfg.SplitAccountIds {
+		if id == uuid.Nil {
+			return nil, fmt.Errorf("invalid or mal-formatted UUID in splitAccountIds config")
+		}
 	}
 
 	return &cfg, nil
diff --git a/go.mod b/go.mod
index 57a32ab..6b21e2a 100644
--- a/go.mod
+++ b/go.mod
@@ -3,11 +3,13 @@ module github.com/samshadwell/split-ynab
 go 1.21
 
 require (
+	github.com/google/uuid v1.4.0
 	github.com/oapi-codegen/runtime v1.1.0
+	go.uber.org/zap v1.26.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
 require (
 	github.com/apapsch/go-jsonmerge/v2 v2.0.0 // indirect
-	github.com/google/uuid v1.4.0 // indirect
+	go.uber.org/multierr v1.11.0 // indirect
 )
diff --git a/go.sum b/go.sum
index c1b836a..f3f4eaa 100644
--- a/go.sum
+++ b/go.sum
@@ -17,6 +17,12 @@ github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
 github.com/stretchr/testify v1.8.4 h1:CcVxjf3Q8PM0mHUKJCdn+eZZtm5yQwehR5yeSVQQcUk=
 github.com/stretchr/testify v1.8.4/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
+go.uber.org/goleak v1.2.0 h1:xqgm/S+aQvhWFTtR0XK3Jvg7z8kGV8P4X14IzwN3Eqk=
+go.uber.org/goleak v1.2.0/go.mod h1:XJYK+MuIchqpmGmUSAzotztawfKvYLUIgg7guXrwVUo=
+go.uber.org/multierr v1.11.0 h1:blXXJkSxSSfBVBlC76pxqeO+LN3aDfLQo+309xJstO0=
+go.uber.org/multierr v1.11.0/go.mod h1:20+QtiLqy0Nd6FdQB9TLXag12DsQkrbs3htMFfDN80Y=
+go.uber.org/zap v1.26.0 h1:sI7k6L95XOKS281NhVKOFCUNIvv9e0w4BF8N3u+tCRo=
+go.uber.org/zap v1.26.0/go.mod h1:dtElttAiwGvoJ/vj4IwHBS/gXsEu/pZ50mUIRWuG0so=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
diff --git a/main.go b/main.go
index d006677..a7f9906 100644
--- a/main.go
+++ b/main.go
@@ -2,59 +2,208 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
+	"math/rand"
 	"net/http"
 	"os"
+	"slices"
+	"time"
 
+	"github.com/google/uuid"
+	"github.com/oapi-codegen/runtime/types"
 	"github.com/samshadwell/split-ynab/storage"
 	"github.com/samshadwell/split-ynab/ynab"
+	"go.uber.org/zap"
 )
 
 const ynabServer = "https://api.ynab.com/v1"
 
 func main() {
+	logger, err := zap.NewDevelopment()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error while creating logger: %v\n", err)
+		os.Exit(1)
+	}
+	defer func() {
+		err = errors.Join(err, logger.Sync())
+	}()
+
 	config, err := LoadConfig()
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "%v\n", err)
+		logger.Error("failed to load config", zap.Error(err))
 		os.Exit(1)
 	}
 
-	client, err := constructClient(config.YnabToken)
+	client, err := constructYnabClient(config.YnabToken)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Error while constructing client: %v\n", err)
+		logger.Error("failed to construct client", zap.Error(err))
 		os.Exit(1)
 	}
 
 	storage := storage.NewLocalStorageAdapter()
-	// Ignore error return, we can use the default value of 0 in case of error.
+	// In case of error we'll process more transactions than we need to, but don't need to exit.
 	serverKnowledge, _ := storage.GetLastServerKnowledge(config.BudgetId)
 
-	transactionsResponse, err := client.GetTransactionsByAccountWithResponse(
-		context.TODO(),
-		config.BudgetId,
-		config.SplitAccountIds[0],
-		&ynab.GetTransactionsByAccountParams{
-			LastKnowledgeOfServer: &serverKnowledge,
-		},
-	)
+	transactionsResponse, err := fetchTransactions(logger, config.BudgetId, serverKnowledge, client)
+	if err != nil {
+		logger.Error("failed to fetch transactions from YNAB", zap.Error(err))
+		os.Exit(1)
+	}
+
+	updatedServerKnowledge := transactionsResponse.JSON200.Data.ServerKnowledge
+	filteredTransactions := filterTransactions(transactionsResponse.JSON200.Data.Transactions, config)
+	logger.Info("finished filtering transactions", zap.Int("count", len(filteredTransactions)))
+
+	if len(filteredTransactions) == 0 {
+		logger.Info("no transactions to update, exiting")
+		// Ignore errors since we're exiting anyway
+		_ = storage.SetLastServerKnowledge(config.BudgetId, updatedServerKnowledge)
+		os.Exit(0)
+	}
+
+	updatedTransactions := splitTransactions(filteredTransactions, config)
+
+	err = updateTransactions(logger, config.BudgetId, updatedTransactions, client)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Error while getting transactions: %v\n", err)
+		logger.Error("failed to update transactions in YNAB", zap.Error(err))
 		os.Exit(1)
 	}
 
-	newKnowledge := transactionsResponse.JSON200.Data.ServerKnowledge
-	fmt.Printf("Transaction count: %v, New server knowledge: %v\n", len(transactionsResponse.JSON200.Data.Transactions), newKnowledge)
-	err = storage.SetLastServerKnowledge(config.BudgetId, newKnowledge)
+	logger.Info("setting server knowledge", zap.Int64("serverKnowledge", updatedServerKnowledge))
+	err = storage.SetLastServerKnowledge(config.BudgetId, updatedServerKnowledge)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Error while setting new server knowledge: %v\n", err)
+		logger.Error("failed to set new server knowledge", zap.Error(err))
 		os.Exit(1)
 	}
+
+	logger.Info("run complete, program finished successfully")
 }
 
-func constructClient(authToken string) (*ynab.ClientWithResponses, error) {
+func constructYnabClient(authToken string) (*ynab.ClientWithResponses, error) {
 	authRequestEditor := func(ctx context.Context, req *http.Request) error {
 		req.Header.Add("Authorization", fmt.Sprintf("Bearer %s", authToken))
 		return nil
 	}
 	return ynab.NewClientWithResponses(ynabServer, ynab.WithRequestEditorFn(authRequestEditor))
 }
+
+func fetchTransactions(
+	logger *zap.Logger,
+	budgetId uuid.UUID,
+	serverKnowledge int64,
+	client *ynab.ClientWithResponses,
+) (*ynab.GetTransactionsResponse, error) {
+	logger.Info("fetching transactions from YNAB",
+		zap.String("budgetId", budgetId.String()),
+		zap.Int64("lastKnowledgeOfServer", serverKnowledge),
+	)
+
+	transactionParams := ynab.GetTransactionsParams{}
+	if serverKnowledge == 0 {
+		// If we don't have any server knowledge, only update transactions from the last 30 days
+		today := time.Now()
+		thirtyDaysAgo := today.AddDate(0, 0, -30)
+		transactionParams.SinceDate = &types.Date{Time: thirtyDaysAgo}
+	} else {
+		transactionParams.LastKnowledgeOfServer = &serverKnowledge
+	}
+
+	transactionsResponse, err := client.GetTransactionsWithResponse(
+		context.TODO(),
+		budgetId.String(),
+		&transactionParams,
+	)
+	if err != nil {
+		return nil, err
+	}
+	statusCode := transactionsResponse.StatusCode()
+	if statusCode != http.StatusOK {
+		return nil, fmt.Errorf("non-200 response from YNAB when fetching transactions: %v", statusCode)
+	}
+
+	logger.Info("successfully fetched transactions from YNAB",
+		zap.Int("count", len(transactionsResponse.JSON200.Data.Transactions)),
+	)
+
+	return transactionsResponse, err
+}
+
+func filterTransactions(transactions []ynab.TransactionDetail, cfg *config) []ynab.TransactionDetail {
+	var filtered []ynab.TransactionDetail
+	for _, t := range transactions {
+		if t.Amount == 0 || t.Cleared == ynab.Reconciled || len(t.Subtransactions) != 0 {
+			// Skip if zero amount, reconciled, or already split
+			continue
+		}
+
+		if slices.Contains(cfg.SplitAccountIds, t.AccountId) {
+			filtered = append(filtered, t)
+			continue
+		}
+	}
+	return filtered
+}
+
+func splitTransactions(transactions []ynab.TransactionDetail, cfg *config) []ynab.SaveTransactionWithId {
+	split := make([]ynab.SaveTransactionWithId, len(transactions))
+	for i, t := range transactions {
+		// Copy the fields we need pointers to
+		id := t.Id
+		categoryId := t.CategoryId
+
+		totalCentiUnits := t.Amount / 10
+		paidAmount := (totalCentiUnits / 2) * 10
+		owedAmount := paidAmount
+		if paidAmount+owedAmount != t.Amount {
+			extra := t.Amount - (paidAmount + owedAmount)
+			// Randomly assign the extra cent to one of the two people
+			if rand.Intn(2) == 0 {
+				paidAmount += extra
+			} else {
+				owedAmount += extra
+			}
+		}
+
+		split[i] = ynab.SaveTransactionWithId{
+			Id:         &id,
+			CategoryId: nil,
+			Subtransactions: &[]ynab.SaveSubTransaction{
+				{
+					Amount:     paidAmount,
+					CategoryId: categoryId,
+				},
+				{
+					Amount:     owedAmount,
+					CategoryId: &cfg.SplitCategoryId,
+				},
+			},
+		}
+	}
+
+	return split
+}
+
+func updateTransactions(
+	logger *zap.Logger,
+	budgetId uuid.UUID,
+	updatedTransactions []ynab.SaveTransactionWithId,
+	client *ynab.ClientWithResponses,
+) error {
+	logger.Info("updating transactions in YNAB")
+	resp, err := client.UpdateTransactionsWithResponse(
+		context.TODO(),
+		budgetId.String(),
+		ynab.UpdateTransactionsJSONRequestBody{
+			Transactions: updatedTransactions,
+		},
+	)
+	if err != nil {
+		return err
+	}
+	if resp.StatusCode() != http.StatusOK {
+		return fmt.Errorf("non-200 response from YNAB when updating transactions: %v", resp.StatusCode())
+	}
+	logger.Info("successfully updated transactions in YNAB")
+	return nil
+}
diff --git a/storage/file_storage_adapter.go b/storage/file_storage_adapter.go
index fba9b55..efcfe72 100644
--- a/storage/file_storage_adapter.go
+++ b/storage/file_storage_adapter.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 
+	"github.com/google/uuid"
 	"gopkg.in/yaml.v3"
 )
 
@@ -12,8 +13,8 @@ type localStorageAdapter struct{}
 const storageFile = "storage.yml"
 
 type budgetData struct {
-	BudgetId            string `yaml:"budgetId"`
-	LastServerKnowledge int64  `yaml:"lastServerKnowledge"`
+	BudgetId            uuid.UUID `yaml:"budgetId"`
+	LastServerKnowledge int64     `yaml:"lastServerKnowledge"`
 }
 
 // Creates a StorageAdapter which stores data in a yaml file. Intended mostly for prototyping or running in environments
@@ -22,8 +23,8 @@ func NewLocalStorageAdapter() StorageAdapter {
 	return &localStorageAdapter{}
 }
 
-func (l *localStorageAdapter) GetLastServerKnowledge(budgetId string) (int64, error) {
-	data, err := l.readData(budgetId)
+func (l *localStorageAdapter) GetLastServerKnowledge(budgetId uuid.UUID) (int64, error) {
+	data, err := l.readData()
 	if err != nil {
 		return 0, err
 	}
@@ -34,14 +35,14 @@ func (l *localStorageAdapter) GetLastServerKnowledge(budgetId string) (int64, er
 		}
 	}
 
-	return 0, fmt.Errorf("No budget found with id %v", budgetId)
+	return 0, fmt.Errorf("no budget found with id %v", budgetId)
 }
 
-func (l *localStorageAdapter) SetLastServerKnowledge(budgetId string, serverKnowledge int64) error {
+func (l *localStorageAdapter) SetLastServerKnowledge(budgetId uuid.UUID, serverKnowledge int64) error {
 	var data []budgetData
 
 	if _, err := os.Stat(storageFile); err == nil {
-		data, err = l.readData(budgetId)
+		data, err = l.readData()
 		if err != nil {
 			return err
 		}
@@ -74,7 +75,7 @@ func (l *localStorageAdapter) SetLastServerKnowledge(budgetId string, serverKnow
 	return encoder.Encode(data)
 }
 
-func (l *localStorageAdapter) readData(budgetId string) ([]budgetData, error) {
+func (l *localStorageAdapter) readData() ([]budgetData, error) {
 	f, err := os.Open(storageFile)
 	if err != nil {
 		return nil, err
diff --git a/storage/storage_adapter.go b/storage/storage_adapter.go
index 6dde5f9..e8b1644 100644
--- a/storage/storage_adapter.go
+++ b/storage/storage_adapter.go
@@ -1,6 +1,8 @@
 package storage
 
+import "github.com/google/uuid"
+
 type StorageAdapter interface {
-	GetLastServerKnowledge(budgetId string) (int64, error)
-	SetLastServerKnowledge(budgetId string, serverKnowledge int64) error
+	GetLastServerKnowledge(budgetId uuid.UUID) (int64, error)
+	SetLastServerKnowledge(budgetId uuid.UUID, serverKnowledge int64) error
 }