How to reliably get timestamp at which the system booted?

First of all, crtime is tricky on Linux. That said, running something like

$ stat -c %z /proc/ 
2014-10-30 14:00:03.012000000 +0100

or

$ stat -c %Z /proc/ 
1414674003

is probably exactly what you need. The /proc file system is defined by the LFS standard and should be there for any Linux system as well as for most (all?) UNIXen.

Alternatively, assuming you don't really need seconds precision, but only need the timestamp to be correct, you can use who:

$ who -b
   system boot  2014-10-30 14:00

From man who: -b, --boot time of last system boot

You can convert that to seconds since the epoch using GNU date:

$ date -d "$(who -b | awk '{print $4,$3}' | tr - / )" +%s
1414674000

Another solution is /proc/stat's btime [1]:

$ cat /proc/stat | grep btime | awk '{print $2}'

Example output:

1583547431

This is a seconds-since-epoch timestamp.

[1] http://man7.org/linux/man-pages/man5/proc.5.html