return same timestamp format for all sensor types

g.getstarttime() was returning timestamps in iso8601 format for Axivity .cwa and GENEActiv .bin files, but it was returning timestamps in POSIX format for all other input formats.

The only caller of g.getstarttime() is get_starttime_weekday_meantemp_truncdata(), and it isn't handling  iso8601 format correctly.

So now g.getstarttime() will return POSIX timestamps for all sensor types.

NEWS.md

@@ -1,3 +1,7 @@
+# CHANGES IN GGIR VERSION 3.0-3
+
+- Part 1: Fix bug where on machines with GMT timezone and R >= 4.3.0, for GENEActiv .bin files, the starting timestamps of M$metalong and M$metashort were truncated to midninght #1000
+
 # CHANGES IN GGIR VERSION 3.0-2
 
 - Part 2: Fix bug that caused part 2 to struggle with corrupt ActiGraph .gt3x files #972

R/g.getstarttime.R

@@ -4,10 +4,8 @@ g.getstarttime = function(datafile, P, header, mon, dformat, desiredtz, configtz
   if (mon == MONITOR$AXIVITY && dformat == FORMAT$CWA) {
     starttime = P$data[1,1]
     starttime = as.POSIXlt(starttime, tz = desiredtz, origin = "1970-01-01")
-    starttime = POSIXtime2iso8601(starttime, tz = desiredtz)
   } else if (mon == MONITOR$GENEACTIV && dformat == FORMAT$BIN) {
     starttime = as.POSIXlt(P$data.out$time[1], tz = desiredtz, origin = "1970-01-01")
-    starttime = POSIXtime2iso8601(starttime, tz = desiredtz)
   } else if (dformat == FORMAT$CSV && (mon == MONITOR$ACTIGRAPH || mon == MONITOR$AXIVITY || mon == MONITOR$VERISENSE)) {
     if (mon == MONITOR$ACTIGRAPH || mon == MONITOR$VERISENSE) {
       tmph = read.csv(datafile, nrow = 8, skip = 1)