GMT to Local Time Routine

[johncouture]

Interesting ideas.

Subtracting 8 hours does seem like a solution except I will need to take into account adding or subtracting enough MINUTES so that I accurately keep track of crossing midnight. The reason I say minutes is that some places have a local time with a 30 minute difference.

Using Epoch time is interesting and I partially did that in the last couple of hours. However, I was worried that the number of minutes in a year exceeded the size of a WORD variable (LONG variables are part of the PIC 18F series and I am using a 16F).

Later on today and with some more forum searching, I came up with a partial solution.

Working on the premise that a large display clock only displays the hour and the minute, you only need to worry about the last 24 hours. Thus, you do not need to worry about a DAY OF MONTH, MONTH or YEAR change. That greatly shortens the code. I'll tackle those at a later date. Thus, my largest number would be 1440 minutes which will fit in a WORD variable.

Next I needed to convert from BCD to DEC and back. Remember I am using BCD because the RTC stores it that way. I am using DEC to do the epoch calculations. A couple of posts on this forum provided some important clues.

To convert BCD to DEC (I love the simplicity of it!)

myDEC = ((myBCD >> 4) * 10) + (myBCD & $0F)

To convert the DEC to BCD, the forum provided another clue by using the DIG mathematical operator (pg 74 in manual, sec 3.1.9):

myBCD = ((myDEC DIG 1) * 16) + (myDEC DIG 0)

Again, I did not invent these, they were found on the forum.

CONVERTING TO LOCAL TIME:

Remember I only have to do the hours and minutes for now (I have already burned too many brain cells on this project today, I'll tackle the rest at another date)

' ==== First I read the RTC and to bring the BCD values into ClkYear,ClkMon, etc

' ==== Next I convert all those BCD values to DEC so I can do Epoch calculations

DstYear =((ClkYear >> 4) * 10) + (ClkYear & $0F) ' last two digits of year
DstMon = ((ClkMon >> 4) * 10) + (ClkMon & $0F) ' Month starting at 01
DstDte = ((ClkDte >> 4) * 10) + (ClkDte & $0F) ' Date of the month, i.e. the 13th
DstDOW = ClkDOW ' Day of week (I don't use this)
DstHrs = ((ClkHrs >> 4) * 10) + (ClkHrs & $0F) ' 24 hour clock
DstMin = ((ClkMin >> 4) * 10) + (ClkMin & $0F) ' minutes
DstSec = ((ClkSec >> 4) * 10) + (ClkSec & $0F) ' seconds

' ==== Now I make a mini epoch :-) of the last 24 hours
' ==== DstTime contains the number of minutes in minutes, hours and Day of the Month (DstDte)

DstTime = DstMin + (DstHrs * 60) ' number of minutes in minutes and hours
DstTime = DstTime + (DstDte * 1440) ' At this point you have a mini Epoch of the number of minutes in this month

' ==== Next subtract out the number of minutes from GMT.

DstTime = DstTime - DstOffset ' for me in California, this is 7 hours * 60 minutes or 420 minutes


' ==== This won't work for most of the world, I just need to get something working for
' ==== where I live in California U.S.A. I'll work out the rest of the world later.
' ==== Also, this does not automatically change from DST to ST..... later, later, later

' ==== Finally, convert your mini Epoch to decimal times

DstDte = DstTime / 1440 ' Day of the month
DstHrs = (DstTime // 1440) / 60 ' Military Hours
DstMin = (DstTime // 60) ' Minutes

Hope that helps. Now I need to finish my project and then come back to this and make it into a usable set of functions. :-)

Thank you all for the ideas!

To-Do Ideas:
1) Check to see what happens at month crossover. Need to allocate for months with different number of days and Feb with 29 days
2) Once a month changes, check to see if year changed.
3) Need a configuration file that has the name of the timezone, start of DST, end of DST and offset with plus/minus in minutes. Dates will likely be different from year to year (see http://www.timeanddate.com/time/dst/ )
4) Need to store config file. How about in RTC which has 56 bytes of battery backed memory.
5) When year changes, go look for config file and update config
6) Tie all of this into a tutorial on how to interface the ESP8266 wifi module (about $3) so you don't have to use WWVB, GPS, etc.
7) How to determine your approx location??? Does wifi module know physical location of gateway? That way you could set config automagically
8) Is there an easy way to do BCD arithmetic?

=== End of Message ===

[johncouture]

Oh, a couple of side notes:

1) technically you do not need to worry about seconds because GMT to local time share the same seconds.

2) if you want to ignore those places that do partial hours, you can eliminate minutes too. But be nice to India, Australia, Newfoundland, etc. :-)

[Art]

In Eastern Australia we are +10.

You are lucky if you are only displaying 24 hour time, and not the date,
or you need a full calendar all the way to checking leap years.
Just the one hour ahead the hour before midnight on a New Year’s Eve,
would make the entire date and day of week incorrect otherwise.

The calendar code I have posted here has done fine on that with my testing so far.
It is also the thing that tells you if any of it’s fields changed.

[johncouture]

Thank you Art! I'll look at it.

[gadelhas]

In Eastern Australia we are +10.

You are lucky if you are only displaying 24 hour time, and not the date,
or you need a full calendar all the way to checking leap years.
Just the one hour ahead the hour before midnight on a New Year’s Eve,
would make the entire date and day of week incorrect otherwise.

Thats why its better working with EpochTime.

[Art]

How do you not still need a calendar?.. You can calculate all you want from an epoch moment,
how do you then know the day of week, or if a date such as the 45th of June 1955 is invalid?
The calendar still counts from an epoch... you need to begin with a known leap year.

[gadelhas]

How do you not still need a calendar?.. You can calculate all you want from an epoch moment,
how do you then know the day of week, or if a date such as the 45th of June 1955 is invalid?
The calendar still counts from an epoch... you need to begin with a known leap year.

Hi Art;

The Epoch/Unix time ( 32 bits ) its valid since 1970 to 1938. Probably it will be changed to 64bits and then it will be valid longer than the age of the universe. ( Probably )

Several Industrial systems and Computer systems uses this aproach.
You don't need a calender for nothing, you can calculate everything from epoch time, including day of week, and also if a some date is valid or not.
By the way , to calculate day of week, or leap year you don't even need epoch time.

References;

Epoch/Unix time - https://en.wikipedia.org/wiki/Unix_time
Day of week calculation - https://en.wikipedia.org/wiki/Determ...ay_of_the_week

Calculating leap year. ( the algorith that i use ) You can find others;
if((year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0))
{
isleapyear;
}
else
{
isnotleapyear;
}

Videos;

[Art]

This sounds like exactly what I did.. an maybe just that I call the code a calendar.
Except that I have a lookup table of leap years and start the algorithm from the closest to current.
Actually, thinking again, it might have been dates known to be a Monday on the first day.