Do not use time.Date to convert BS dates

I was reading through the code and saw this piece of code.

Line 37 in 065a8fb

return toTime(year, month, days)

The question that popped in my head right away is how can we possibly get the right Bikram Sambat dates if we use the inbuilt time.Date function which possibly follows the Gregorian calendar.

The Gregorian calendar never exceeds 31 days. However, the Bikram Sambat calendar does.

I quickly checked the Nepali calendar and found June 15, 2019 to be जेठ 32, 2076. I tried converting this date and here are the results.

Wrong Results: ❎

nepcal conv adtobs 06-15-2019
असार 3, 2076 शनिबार

nepcal conv adtobs 06-13-2019
असार 1, 2076 बिहिबार

Right Results: ✅

nepcal conv adtobs 06-18-2019
असार 3, 2076 मंगलबार

nepcal conv adtobs 06-17-2019
असार 2, 2076 सोमबार

Let's take June 15, 2019 as an example. Supposedly, the conversion is right and we get year, month, days to be 2076, 02, 32 which translates to जेठ 32, 2076.

Now when we pass this result to toTime it will try to convert these dates to Gregorian calendar. If you take a look at the 2076 Gregorian calendar, the month of February (02) has only 29 days. Amazingly, what time.Date tries to do here is append the excess of 3 days and add it to the next month. This results in the output of 2076, 03, 03 which is असार 3, 2076.

There might be plethora of dates that don't convert well. 😛

Thanks for finding the issue, and for the detailed breakdown as well.

If I understand correctly, the problem is as follows:

If a B.S. date (say जेठ 32) is represented as 02/32 (which is the case right now), toTime will (obviously) interpret it as Feb 32 which is an overflow by 32 - 29 = 3 days. So it's reinterpreted by the time package to be March 3 which is interpreted by nepcal to be असार 3.

This is problematic for sure. I guess the fix is to create a BSDate struct which holds these values and return that. This struct could also hold the eventual toAD() method. Using this struct would also imply changes to the main package which works with time.Date. I think this is the scope of what needs to be done - shouldn't be too difficult to make it work with the new types.

Thanks again!

For completeness, the following test cases should pass:

AD Date	Current BS Output	Expected Output
June 15, 2019	असार 3, 2076	जेठ 32, 2076
June 13, 2019	असार 1, 2076	जेठ 30, 2076

Presumably, it fails because the following happens inside the time package.

yy, mm, dd = func() { .. }()

if adMonth(mm).hasLessDaysThan(dd) {
  mm = mm + 1
  dd = dd - numberOfDaysInAdMonth(mm) // overflow
}

Fixed and released in v0.5.0!