Working with CFAbsoluteTime and epoch time in shell

CFAbsoluteTime_Unix_Epoch.sh

#work with CFAbsoluteTime in shell
#https://developer.apple.com/documentation/corefoundation/cfabsolutetime
#https://developer.apple.com/documentation/corefoundation/1543542-cfabsolutetimegetcurrent

#get epoch time for CFAbsoluteTime 0 (978307200)
#same as the constant kCFAbsoluteTimeIntervalSince1970 in https://github.com/opensource-apple/CF/blob/master/CFDate.c
/bin/date -j -f "%b %d %T %Z %Y" "Jan  1 00:00:00 GMT 2001" "+%s" 

#Now in CFAbsoluteTime
echo $(( $(date +"%s") - 978307200 ))

#convert CFAbsoluteTime to epoch
#use parameter expansion to truncate mantissa
CFAbsoluteTime=""
echo $(( 978307200 + ${CFAbsoluteTime%.*} ))

#convert CFAbsoluteTime time to normal date stamp
#use parameter expansion to truncate mantissa
CFAbsoluteTime=""
date -r $(( 978307200 + ${CFAbsoluteTime%.*} ))
#in UTC
date -ur $(( 978307200 + ${CFAbsoluteTime%.*} ))