# Help understanding --update

**URL:** <https://forum.rclone.org/t/help-understanding-update/28937>\
**Category:** Help and Support\
**Created:** [January 28, 2022, 10:17am UTC](https://forum.rclone.org/t/help-understanding-update/28937 "2022-01-28T10:17:48Z")\
**Posts on this page:** 1\
**Showing post:** 6

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**Author:** ![ncw](https://forum.rclone.org/user_avatar/forum.rclone.org/ncw/32/18_2.png) [@ncw](https://forum.rclone.org/u/ncw)\
**Post date:** [January 31, 2022, 5:21pm UTC](https://forum.rclone.org/t/help-understanding-update/28937/6 "2022-01-31T17:21:04Z")

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Thanks for your update 🙂

I re-read the code again, and realised I'd got the cases the wrong way round, so I've taken your docs and tweaked them with the new info.

OK?

### -u, --update

This forces rclone to skip any files which exist on the destination  
and have a modified time that is newer than the source file.

This can be useful in avoiding needless transfers when transferring to  
a remote which doesn't support modification times directly (or when  
using `--use-server-modtime` to avoid extra API calls) as it is more  
accurate than a `--size-only` check and faster than using  
`--checksum`. On such remotes (or when using `--use-server-modtime`)  
the time checked will be the uploaded time.

If an existing destination file has a modification time older the source  
file's, it will be updated if the sizes are different. If the sizes  
are the same, it will be updated if the checksum is different or not  
available.

If an existing destination file has a modification time equal (within  
the computed modify window) to the source file's, it will be updated  
if the sizes are different. The checksum will not be checked in this  
case unless the `--checksum` flag is provided.

In all other cases the file will not be updated.

Consider using the `--modify-window` flag to compensate for time skews  
between the source and the backend, for backends that do not support  
mod times, and instead use uploaded times. However, if the backend  
does not support checksums, note that sync'ing or copying within the  
time skew window may still result in additional transfers for safety.

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