= The year 2038 problem =
All 32-bit kernels to date use a signed 32-bit time_t type, which can only represent time until January 2038. Since embedded systems running 32-bit Linux are going to survive beyond that date, we have to change all current uses, in a backwards compatible way.

== User space interfaces ==
We will likely keep the 32-bit time_t in all user space interfaces that currently use it, but add new interfaces with a 64-bit timespec or another type that can represent later times. Most importantly that impacts system calls, but also specific ioctl commands and a few other interfaces. User space programs have to be recompiled to use the new interfaces, and the policy whether to use the old or the time time is left to the C library. While that policy is a complex topic itself, we don't cover it here.

=== System calls ===
==== interfaces that uses relative time_t/timespec/timeval ====
These can stay compatible, but we'd have to use a different type if we change time_t.

 * nanosleep
 * select/pselect/poll/ppoll/epoll
 * getrusage
 * sched_rr_get_interval
 * sigtimedwait
 * alarm

==== interfaces that don't make sense for times in the past ====
Here, we are relatively free to change the start of the epoch in the kernel but convert to something else on the user space boundary. One possibility is to scale them to boot time and use ktime_t in the kernel.

 * getitimer/setitimer
 * timer_settime/timer_gettime
 * gettimeofday/settimeofday
 * adjtimex
 * clock_gettime/clock_settime/clock_adjtime/clock_nanosleep
 * time/stime
 * sysv ipc (msg, sem, shm)
 * sysinfo

==== interfaces that require absolute times ====
These absolutely have to use something better than time_t both in user space and in the kernel so we can deal with old files. A lot of file systems need to be fixed as well so we can actually store the times, regardless of whether we are running a 32 or 64 bit kernel.

 * stat/lstat/fstatat/
 * utime/utimes/futimesat

=== ioctl ===
There are numerous ioctl commands using a time argument. This list is incomplete

 * audio time stamps
 * v4l time stamps
 * input event time stamps
 * socket time stamps
 * ...

=== memory mapped packet sockets ===
Socket timestamps are exported to user space using a memory mapped interface defined in include/uapi/linux/if_packet.h. There are currently three versions of this interface, all use a 32-bit time type. We will likely need a version 4 to solve this.

== File systems ==
Each file system stores its file modification times in its own format on disk, and a lot of them have the same problem.

|| '''file system''' || '''time type''' || '''expiration year''' ||
|| 9p || u32 seconds || 2106 ||
|| adfs || 40-bit cs since 1900 || 2248 ||
|| affs || u32 days/mins/(secs/50) || 11760870 ||
|| afs ||  s32 seconds (bug)||2038||
|| autofs4 || internal ||  ||
|| befs ||  48-bit seconds||  never||
|| bfs ||  u32 seconds||  2106||
|| btrfs || 64-bit seconds, 32-bit ns || never ||
|| cachefiles || internal ||  ||
|| ceph || unsigned 32-bit second/ns || 2106 ||
|| cifs (smb) || 7-bit years since 1980 || 2107 ||
|| cifs (modern) || 64-bit 100ns since 1601 || 30328 ||
|| coda || timespec ioctl || 2038 ||
|| configfs || internal ||  ||
|| cramfs || fixed || 1970 ||
|| debugfs || internal ||  ||
|| devpts || internal ||  ||
|| dlm || internal ||  ||
|| ecryptfs || internal ||  ||
|| efivarfs || internal ||  ||
|| efs || u32 seconds || 2106 ||
|| exofs || s32 seconds || 2038 ||
|| exportfs || internal ||  ||
|| ext2 || u32 seconds || 2106 ||
|| ext3 || u32 seconds || 2106 ||
|| ext4 || 34 bit seconds / 30-bit ns || 2514 ||
|| f2fs || 64-bit seconds / 32-bit ns || never ||
|| fat || 7-bit years since 1980, 2s resolution || 2107 ||
|| freevxfs ||  u32 seconds/u32 microseconds||  2106||
|| fscache || internal ||  ||
|| fuse || 64-bit second/32-bit ns || never ||
|| gfs2 ||  u64 seconds/u32 ns||  never||
|| hfs || u32 seconds since 1904 || 2040 ||
|| hfsplus || u32 seconds since 1904 || 2040 ||
|| hostfs ||  timespec||  2038||
|| hpfs ||  u32 seconds||  2106||
|| hppfs ||  internal||  ||
|| hugetlbfs || internal ||  ||
|| isofs || 'char' year since 1900||  2028 (!)||
|| jbd || internal ||  ||
|| jbd2 || internal ||  ||
|| jffs2 || u32 seconds || 2106 ||
|| jfs ||  u32 seconds/ns||  2106||
|| kernfs || internal ||  ||
|| lockd || internal ||  ||
|| logfs || u64 ns || 2554 ||
|| minix ||  u32 seconds||  2106||
|| ncpfs ||  7-bit year since 1980||  2107||
|| nfsv2,v3 || u32 seconds/ns || 2106 ||
|| nfsv4 || u64 seconds/u32 ns || never ||
|| nfs_common || internal ||  ||
|| nfsd || u32 seconds/ns || 2106 ||
|| nilfs2 ||  u64 seconds/u32 ns||  never||
|| nls || internal ||  ||
|| notify || internal ||  ||
|| ntfs || 64-bit 100ns since 1601 || 30828 ||
|| ocfs2 || 34-bit seconds/30-bit ns || 2514 ||
|| omfs ||  64-bit milliseconds||  never||
|| openpromfs || internal ||  ||
|| proc || internal ||  ||
|| pstore || internal ||  ||
|| qnx4 ||  u32 seconds||  2106||
|| qnx6 ||  u32 seconds||  2106||
|| quota || internal ||  ||
|| ramfs || internal ||  ||
|| reiserfs || u32 seconds || 2106 ||
|| romfs || fixed || 1970 ||
|| squashfs || u32 seconds || 2106 ||
|| sysfs || internal ||  ||
|| sysv ||  u32 seconds||  2106||
|| ubifs || u64 second/u32 ns || never ||
|| udf || u16 year || 2038 ||
|| ufs1 ufs2 ||u32 seconds u64 seconds/u32 nanoseconds ||2106 never ||
|| xfs || u32 seconds/ns || 2106 ||