DayName giving incorrect days with TimeZoneConvert

Question

So I used TimeZoneConvert to get the time in GMT:

TimeZoneConvert[Now,"GMT"]

It outputs a date object that says Tuesday 14 March 2017 02:45:08 GMT.

But when I then try:

DayName @ TimeZoneConvert[Now, "GMT"]

I get Monday, even though the date object is a Tuesday. What's going on here?

Answer 1

Bug Confirmed by Wolfram Research

My time zone is GMT+8, which is used in this answer.

I can only explain how you get the incorrect answer.

First we need to read the definitions to trace the evaluation. The powerful tool I used is GeneralUtilities`PrintDefinitions. I will just show the main steps.

First store that converted date object in t, t = TimeZoneConvert[Now,"GMT"].

Check what's inside

t//InputForm
DateObject[{2017, 3, 14}, 
 TimeObject[{5, 58, 23.634473}, 
  TimeZone -> "GMT"], 
 TimeZone -> "GMT"]

From the definitions of DayName,

DataPaclets`CalendarDataDump`iDayName[t]
(*Monday*)

Then,

DataPaclets`CalendarDataDump`doDayName[t]
(*Monday*)

Then,

DataPaclets`CalendarDataDump`ToInternalDate[{"Gregorian", t}, DayName]
(*{"Gregorian", 2017, 3, 13, 21, 58, 23.6345}*)

Here we noticed that this function AGAIN subtracted 8 hours from the date, which is incorrect. Its code mainly contains two parts, the first is roughly

System`DateObjectDump`$tempDate = t;
System`DateObjectDump`correctTimeZoneOffset[
 System`DateObjectDump`iObjectTimeZone[t]]
(*I added the first line to make the code works, otherwise it returns $Failed, I got the value of $tempdate from tracing*)

This returns 0, which is the time zone for this date. The return value of this function is

DateList[Join[PadRight[date, 3, 1], First @ time], TimeZone -> tz]

which is

DateList[Join[PadRight[{2017, 3, 14}, 3, 1], 
  First @TimeObject[{5, 58, 23.634473}, TimeZone -> "GMT"]], 
 TimeZone -> 0]

in our case. Unfortunately, from the docs of DateList

DateList[TimeZone->z] gives the date and time inferred for time zone z by assuming that your computer is set for the time zone specified by $TimeZone. »

It is assuming the input, {2017, 3, 14, 5, 58, 23.6345} is the local time, and converts it to TimeZone -> 0, so then another 8 hours is subtracted from it, finally we get

{2017, 3, 13, 21, 58, 23.6345}