move contribution from ledger to ana

ana/window.go

@@ -1,6 +1,9 @@
 package ana
 
-import "time"
+import (
+	"fmt"
+	"time"
+)
 
 func predictionTimes(window time.Duration) []time.Time {
 	result := []time.Time{}
@@ -11,3 +14,23 @@ func predictionTimes(window time.Duration) []time.Time {
 	}
 	return result
 }
+
+func mustDateToTime(s string) time.Time {
+	result, err := dateToTime(s)
+	if err != nil {
+		panic(err)
+	}
+	return result
+}
+
+func dateToTime(s string) (time.Time, error) {
+	for _, layout := range []string{
+		"2006-01-02",
+		"2006-01",
+	} {
+		if t, err := time.ParseInLocation(layout, s, time.Local); err == nil {
+			return t, err
+		}
+	}
+	return time.Time{}, fmt.Errorf("no layout matching %q", s)
+}