Fix reading of Igor DateTime

* and add many more tests
* main problem was the handling of daylight saving offsets
* a different result is required when offsets are enforced
* also added tz to all tests since these are otherwise not reproducible

DESCRIPTION

@@ -16,7 +16,8 @@ Description: Provides function to read data from the 'Igor Pro' data analysis
     https://github.com/SilverLabUCL/NeuroMatic for details.
 Imports:
     bitops,
-    tools
+    tools,
+    timechange
 Suggests:
     spelling,
     testthat

R/ReadIgorBinary.R

@@ -338,12 +338,26 @@ read.pxp<-function(pxpfile,regex,ReturnTimeSeries=FALSE,Verbose=FALSE,
   i
 }
 
-igor_date_origin<-as.numeric(ISOdate(1904,1,1,hour=0,tz=""))
-
-.convertIgorDate<-function(dateval){
-  dateval=dateval+igor_date_origin
-  class(dateval)<-"POSIXct"
-  dateval
+# internal function to convert Igor dates
+# these are expressed in seconds since 1904-01-01 in the local timezone
+# note that we provide a tz argument to ensure that tests give the same
+# results regardless of the local timezone of the test machine
+.convertIgorDate<-function(dateval, tz=""){
+  igor_origin=ISOdatetime(1904,1,1,hour=0,min = 0, sec=0, tz=tz)
+  igor_origin_utc=ISOdatetime(1904,1,1,hour=0,min = 0, sec=0, tz='GMT')
+  origin_offset=as.numeric(igor_origin)-as.numeric(igor_origin_utc)
+
+  dateval=dateval+as.numeric(igor_origin)
+  res=as.POSIXct(dateval, tz = tz)
+  # this takes the same clock time (ie hms) but switches time zone, thereby
+  # changing the moment in time
+  res_utc=timechange::time_force_tz(res, tz = 'GMT')
+  res_offset=as.numeric(res)-as.numeric(res_utc)
+
+  # DST correction
+  dst_correction=origin_offset-res_offset
+  res=res-dst_correction
+  res
 }
 
 # enum PackedFileRecordType {

tests/testthat/test-ibw.R

@@ -25,13 +25,26 @@ test_that("Read Igor v2 file", {
     })
 
 test_that("Processing of dates", {
-  expect_equivalent(.convertIgorDate(3444214517),as.POSIXct("2013-02-20 14:15:17"))
+  # when in GMT timezone (Banjul, The Gambia) or BST Igor says
+  # print secs2Time(3444214517, 3) 
+  # 14:15:17 
+  expect_equivalent(.convertIgorDate(3444214517, tz="GMT"),as.POSIXct("2013-02-20 14:15:17", tz = 'GMT'))
+  expect_equivalent(.convertIgorDate(3444214517, tz="GMT"),as.POSIXct("2013-02-20 14:15:17", tz = 'GMT'))
+  expect_equivalent(.convertIgorDate(3444214517, tz="Europe/London"),as.POSIXct("2013-02-20 14:15:17", tz = 'Europe/London'))
   # from nm20120811c1_016.pxp 
   # current implementation says 13:37 on my laptop, lmb cluster and mac in PDT
   # but mtime of file and embedded time string is 12:37 
   # and this is also what I get on a sunos machine in PDT
   # DISABLED due to OS dependent results that I cannot fix
-  expect_equivalent(.convertIgorDate(3427533421),as.POSIXct("2012-08-11 13:37:01"))
+  expect_equivalent(.convertIgorDate(3427533421, tz = 'GMT'),as.POSIXct("2012-08-11 12:37:01", tz='GMT'))
+  expect_equivalent(.convertIgorDate(3427533421, tz = 'Europe/London'),as.POSIXct("2012-08-11 12:37:01", tz='Europe/London'))
+
+  # when in GMT timezone or BST Igor says
+  # print secs2Time(3805612788, 3) 
+  # 10:39:48
+  expect_equivalent(.convertIgorDate(3805612788, tz='GMT'),as.POSIXct("2024-08-04 10:39:48", tz='GMT'))
+  expect_equivalent(.convertIgorDate(3805612788, tz='Europe/London'),as.POSIXct("2024-08-04 10:39:48", tz='Europe/London'))
+  expect_equivalent(.convertIgorDate(3805612788, tz='America/New_York'),as.POSIXct("2024-08-04 10:39:48", tz='America/New_York'))
 })
 
 test_that("Processing of funny null terminated strings", {