How do I convert a UNIX timestamp to ISO in CMake - cmake

Question

I have a UNIX-style timestamp that looks like 1587405820 -0600 which I would like to convert to an ISO style format, something like YYYY-MM-DDTHH:MM:SSZ
CMake has a string(TIMESTAMP ...) command at https://cmake.org/cmake/help/v3.12/command/string.html#timestamp but this only gets me the current time in a formatted string which does not work for my application. I need to be able to convert an existing time into ISO format.
Is there a way to do this?
UPDATE
Based on #squareskittles answer, here's what I ended up with this test which is doing the right thing:
# Check that we get the current timestamp
string(TIMESTAMP TIME_T UTC)
message(STATUS ">>> T1: ${TIME_T}")
# Get the ISO string from our specific timestamp
set(ENV{SOURCE_DATE_EPOCH} 1587405820)
string(TIMESTAMP TIME_T UTC)
unset(ENV{SOURCE_DATE_EPOCH})
message(STATUS ">>> T2: ${TIME_T}")
# Check that we get the current timestamp again correctly
string(TIMESTAMP TIME_T UTC)
message(STATUS ">>> T3: ${TIME_T}")
Which gives me this output:
-- >>> T1: 2020-04-22T15:08:13Z
-- >>> T2: 2020-04-20T18:03:40Z
-- >>> T3: 2020-04-22T15:08:13Z

Answer 1

If you want this function to use a specific time other than than the current time, you can set the environment variable SOURCE_DATE_EPOCH to the UNIX-style timestamp (integer):
# Set the environment variable to a specific timestamp.
set(ENV{SOURCE_DATE_EPOCH} 1587405820)
# Convert to ISO format, and print it.
string(TIMESTAMP MY_TIME)
message(STATUS ${MY_TIME})
prints (for UTC -0600):
2020-04-20T12:03:40
If you need to adjust this time to UTC time, you can add the UTC argument:
set(ENV{SOURCE_DATE_EPOCH} 1587405820)
string(TIMESTAMP MY_TIME UTC)
message(STATUS ${MY_TIME})
prints:
2020-04-20T18:03:40Z
Note: If this SOURCE_DATE_EPOCH variable is used elsewhere in your CMake code, it is best to save the SOURCE_DATE_EPOCH value before modifying it, so it can be set back to its previous value when complete.