## NetCDF time coordinate

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##### Revision as of 13:56, 18 April 2016 by imported>Lang Guenther (englisch text added the first time)

Revision as of 13:56, 18 April 2016 by imported>Lang Guenther (englisch text added the first time)

## Contents

# Introduction

Date, time and calendar plus (optional) use of time intervals.

# Time information in a netCDF file

## Dimensions

dimensions:

- time =
*number of time steps*or*UNLIMITED*; - bnd = 2 ;

Comment: In case the number of time steps is known before-hand, **Bert Jagers** (Deltares) recommends to use a fixed length instead of an UNLIMITED dimension due to performace reasons. But this only works for variables not exceeding 4 GiB.

## Variables

### Coordinate *time*

double time(time) ;

- time:standard_name = "
**time**" ; - time:long_name = "time" ;
- time:units = "
*time units*since YYYY-MM-DD hh:mm:ss*zone*" ; - time:axis = "T"
- time:calendar = "gregorian"
- time:bounds = "timebnd"

Comment:

- For
*time units*the following units are common:*seconds*,*minutes*,*hours*, and*days*.

- Specification of
*zone*is optional (default is UTC). Some examples for other time zones:*zone*= +1:00 for CET (Central European Time), or*zone*= +2:00 for CEST (Central European Summer Time).

- Attribute
*bounds*must be specified, in case (some) data are e.g. mean or integral (sum) values with respect to time. Examples: Daily mean temperature, daily rainfall. These data must possess attribute*cell_methods*with e.g. "time: mean" or "time: sum". See also CF Conventions "cell_methods".

### Time intervals

double timebnd(time,bnd) ;

Comment:

- timebnd(n,1) is equivalent to the
*lower*bound for the n-th time step. - timebnd(n,2) is equivalent to the
*upper*bound for the n-th time step. - Subsequent intervals may not be continous.
- For continuous intervals timebnd(n,2) == timebnd(n+1,1) holds.

# Remarks

- A double precision variable should be used for
*time*to guarantee sufficient accuracy..

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