sqlite> select datetime('2019-03-15 12:00:00', '-1 months');
2019-02-15 12:00:00
sqlite> select datetime('2019-03-28 12:00:00', '-1 months');
2019-02-28 12:00:00
sqlite> select datetime('2019-03-29 12:00:00', '-1 months');
2019-03-01 12:00:00
sqlite> select datetime('2019-03-30 12:00:00', '-1 months');
2019-03-02 12:00:00
sqlite> select datetime('2019-03-31 12:00:00', '-1 months');
2019-03-03 12:00:00
sqlite> select datetime('2019-04-01 12:00:00', '-1 months');
2019-03-01 12:00:00
sqlite> select datetime('2019-03-04 12:00:00', '-1 months');
2019-02-04 12:00:00

bash$ date -j '0315120019'
Fri 15 Mar 2019 12:00:00 AEDT
bash$ date -j -v-1m '0315120019'
Fri 15 Feb 2019 12:00:00 AEDT
bash$ date -j -v-1m '0329120019'
Thu 28 Feb 2019 12:00:00 AEDT
bash$ date -j -v-1m '0330120019'
Thu 28 Feb 2019 12:00:00 AEDT
bash$ date -j -v-1m '0331120019'
Thu 28 Feb 2019 12:00:00 AEDT
____________
Graham Eddy

> On 19 Sep 2019, at 8:39 pm, Andrew Milner <[email protected]> wrote:
> 
> what do sqlite3 and bash return on say 29, 30 and 31 march when one tries to 
> subtract 1 month?
> 
> 
> 
> 
> On Thursday, 19 September 2019 10:51:04 UTC+3, Graham Eddy wrote:
> don’t know enough about specifying timespans within weewx but you can do 
> things like
>   sqlite3> select datetime(dateTime, 'unixepoch’, ’localtime’, ’-1 months’) 
> from archive;
> and
>   bash$ date -v-1m
> ____________
> Graham Eddy
> 

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