Compare date values in expression using govaluate

I am trying to build a rule engine using govaluate: https://github.com/Knetic/govaluate

Here is my code:

package main

import (
	"github.com/Knetic/govaluate"
	"fmt"
)

func main()  {
	expression, err := govaluate.NewEvaluableExpression("first_name == 'Julian' && emp_id == 302 && birth_date >= '2016-12-12' && birth_date <= '2016-12-25'")

	parameters := make(map[string]interface{}, 8)
	parameters["first_name"] = "Julian"
	parameters["emp_id"] = 302
	parameters["birth_date"] = "2016-12-15"
	result, err := expression.Evaluate(parameters)

	if err != nil {
		fmt.Println("Error while evaluating the expression")
	}

	if result.(bool) {
		fmt.Println("Condition is evaluated to true")
	} else {
		fmt.Println("Condition is evaluated to false")
	}
}

While browsing through govaluate Types documentation (https://github.com/Knetic/govaluate/blob/master/MANUAL.md), I noticed this statement "Any string literal (not parameter) which is interpretable as a date will be converted to a float64 representation of that date's unix time."

When i execute the above program, i see an issue with result being null:

Error while evaluating the expression
panic: interface conversion: interface is nil, not bool

My question is whether date value can be passed in as a parameter? As i want to compare date values in runtime, this functionality would be needed.

Is there a way to achieve it? May be my usage pattern is wrong. Please suggest.

Edit:

expression, err := govaluate.NewEvaluableExpression("first_name == 'Julian' && emp_id == 302 && birth_date >= '2016-12-12' && birth_date <= '2016-12-25'")
dateFloat, _ := strconv.ParseFloat("2016-12-15", 64)
parameters["birth_date"] = dateFloat
result, err := expression.Evaluate(parameters)

Changed the date to float format and invoked Evaluate function. Now the condition is evaluated to false. But logically, 2016-12-15 falls between 2016-12-12 and 2016-12-25. The expression should be evaluated to true.

Thanks in advance!

Cross-posting my answer from the associated github issue, for future googlers;

The library doesn't try to parse string parameters as dates - it expects that birth_date will be the Unix() value of that date. This means you'll have to parse the date before passing it to govaluate.

If you're getting birth_date as part of a REST api, I'd suggest settling on one specific date format (such as ISO8601) and doing something like:

var birthDateString string // <-- the birth_date string you get from the request

iso8601 := "2006-01-02T15:04:05Z0700"

birthDate, err := time.ParseInLocation(iso8601, birthDateString, 
time.Local)
parameters["birth_date"] = birthDate.Unix()

Here is the working solution:

package main

import (
	"github.com/Knetic/govaluate"
	"fmt"
	"time"
)

func main()  {
	expression, err := govaluate.NewEvaluableExpression("first_name == 'Julian' && emp_id == 302 && birth_date >= '2016-12-15' && birth_date <= '2016-12-25'")

	parameters := make(map[string]interface{}, 8)
	parameters["first_name"] = "Julian"
	parameters["emp_id"] = 302
	iso8601 := "2006-01-02"
	birthDate, err := time.ParseInLocation(iso8601, "2016-12-15", time.Local)
	parameters["birth_date"] = birthDate.Unix()
	result, err := expression.Evaluate(parameters)

	if err != nil {
		fmt.Println("Error while evaluating the expression")
	}

	if result.(bool) {
		fmt.Println("Condition is evaluated to true")
	} else {
		fmt.Println("Condition is evaluated to false")
	}
}

Thank you Library owner and @RayfenWindspear for helping me out on this! Really appreciate!