# piControl time coordinates
**URL:** https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739
**Category:** Earth System
**Tags:** python, cmip6
**Created:** [29 September 2024 23:38 UTC](https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739 "2024-09-29T23:38:41Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Hasna\_K](https://forum.access-hive.org.au/u/Hasna_K)
#### Post date: [29 September 2024 23:38 UTC](https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739/1 "2024-09-29T23:38:41Z")
Hi,
I have outputs from several CMIP6 models for the piControl experiment, but I noticed that the time coordinates vary between models. For example, one model starts in 1850, another in 0950, and others start in 0001 (I have attached a screenshot below showing the different time coordinates). How should I interpret these varying time coordinates? Also, how can I concatenate the time axis for the piControl, historical, and SSP experiments for further analysis?
Could someone help me with this?
Thanks in advance!
---
### Author:  [@Scott](https://forum.access-hive.org.au/u/Scott)
#### Post date: [30 September 2024 23:51 UTC](https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739/2 "2024-09-30T23:51:18Z")
piControl years should be arbitrary - they represent some ‘pre-industrial’ steady state.
The historical runs are initiated from piControl - each run will come from a different starting point in the piControl data, e.g. ACCESS-ESM r1i1p1 has
```auto
:branch_method = "standard" ;
:branch_time_in_child = 0. ;
:branch_time_in_parent = 21915. ;
```
while r10i1p1 has
```auto
:branch_method = "standard" ;
:branch_time_in_child = 0. ;
:branch_time_in_parent = 87658. ;
```
(I do not know what units this time is in).
SSP runs should continue directly from the end of historical I believe, though check the dates in the file to be sure.
---
### Author:  [@Hasna\_K](https://forum.access-hive.org.au/u/Hasna_K)
#### Post date: [1 October 2024 23:24 UTC](https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739/3 "2024-10-01T23:24:42Z")
Yes, the piControl times are arbitrary (spanning 500 years).
Since the piControl time coords differ across the cmip6 models, i am unable to concatenate them with historical (1850-2014) and SSP(2015-2100) experiments.
---
### Author:  [@Scott](https://forum.access-hive.org.au/u/Scott)
#### Post date: [3 October 2024 04:02 UTC](https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739/4 "2024-10-03T04:02:12Z")
Try something like this - meaningfullness of the time axis - if leap years etc line up properly is left as an exercise
```python
import xarray
import cftime
pic = xarray.open_mfdataset("/g/data/fs38/publications/CMIP6/CMIP/CSIRO/ACCESS-ESM1-5/piControl/r1i1p1f1/Amon/tas/gn/latest/tas_Amon_ACCESS-ESM1-5_piControl_r1i1p1f1_gn_*", use_cftime=True)
his = xarray.open_mfdataset("/g/data/fs38/publications/CMIP6/CMIP/CSIRO/ACCESS-ESM1-5/historical/r1i1p1f1/Amon/tas/gn/latest/tas_Amon_ACCESS-ESM1-5_historical_r1i1p1f1_gn_185001-201412.nc", use_cftime=True)
# Date in 'pic' that 'his' branched from
branch_date = cftime.num2date(his.branch_time_in_parent, his.parent_time_units)
# Correction for 'pic' years to match up with 'his'
year_offset = his.time.dt.year.data[0] - branch_date.year
# Slice out 'pic' before the branch point and override its time axis
pic_before_branch = pic.sel(time=slice(None, branch_date))
pic_before_branch['time'] = pic_before_branch.indexes['time'].map(lambda x: x.replace(year=x.year + year_offset))
# Concatenate the datasets together
extended = xarray.concat([pic_before_branch, his], dim="time")
```
---
### Author:  [@Hasna\_K](https://forum.access-hive.org.au/u/Hasna_K)
#### Post date: [9 October 2024 02:58 UTC](https://forum.access-hive.org.au/t/picontrol-time-coordinates/3739/5 "2024-10-09T02:58:05Z")
Yeah thank you. it almost worked !