count, tabulate, *browse or list variables based on date variable with an elapsed date format

[tig som]

How can I count, tabulate, browse or list variables based on the value of a date variable registered in STATA numeric date format?

Below is my example using _dataex-
The data variable registered in STATA numeric date format is “Birthdate”

I want to browse or count my data by the "Birthdate" variable. However, the STATA outputs are empty values. Even the output of "Birthdate" for -_dataex- is also empty as you see below.
However, the variables are not empty. Below, I also copy and past the "Birthdate" values for your reference.

Birthdate
6/5/2001
10/24/2001
10/24/2001
10/5/2001
6/7/2001
6/7/2001
6/7/2001
3/16/2001
2/19/2001
2/23/2001
2/28/2001
1/22/2001
1/22/2001
1/22/2001
1/15/2001
1/15/2001
1/15/2001
10/11/2001
10/11/2001
10/11/2001
10/19/2001
10/19/2001
1/8/2001
1/8/2001
1/8/2001
1/8/2001
1/29/2001
1/29/2001
1/29/2001
1/29/2001


If I randomly select one value of the "Birthdate" variable and try to count all observations below as presented in the command here (count if Birthdate <2/28/2001), I get 0. However, when I browse and look into the data, there are observations below that values. It seems the function cannot work with this date value. When I do count if Birthdate > 2/28/2001 ( it gives me 30, the number of all observations). This is strange because all observations do not have Birthdate values above 2/28/2001. Moreover, when I do count if Birthdate == 2/28/2001, I get 0 again. This is also strange because as you see in the list of values for the "Birthdate" variable above, one observation has Birthdate== 2/28/2001.

Would you please give me your suggestion,

Thank you.

Below is my example using _dataex-


. dataex ID Birthdate Weight

----------------------- copy starting from the next line -----------------------

* Example generated by -dataex-. For more info, type help dataex
clear
input long ID int Birthdate double Weight
1960010821 15131 .
1960010821 15272 .
1960010821 15272 .
1960010821 15253 .
1960010821 15133 .
1960010821 15133 .
1960010821 15133 .
1960010821 15050 .
1960010821 15025 .
1960010821 15029 .
1960010821 15034 .
1960010821 14997 .
1960010821 14997 .
1960010821 14997 .
1960010821 14990 .
1960010821 14990 .
1960010821 14990 .
1960010821 15259 .
1960010821 15259 .
1960010821 15259 .
1960010821 15267 .
1960010821 15267 .
1960010821 14983 .
1960010821 14983 .
1960010821 14983 .
1960010821 14983 .
1960010821 15004 .
1960010821 15004 .
1960010821 15004 .
1960010821 15004 .
end
format %tdnn/dd/CCYY Birthdate

------------------ copy up to and including the previous line ------------------

Listed 30 out of 30 observations

.
. des Birthdate

Variable Storage Display Value
name type format label Variable label
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Birthdate int %td..

.
. br Birthdate

.
. count if Birthdate < 2/28/2001
0

.
. count if Birthdate == 2/28/2001
0

.
. count if Birthdate > 2/28/2001
30

.
end of do-file

.

[Andrew Musau]

Stata dates are numbers (or referred to as SIF values). So 2/28/2001 is just how the value of such a variable may be displayed. You have convenience functions for what you want, see

help td()

for example. Therefore, you want something like

count if Birthdate < td(28feb2001)

[Nick Cox]

To take Andrew Musau's point further, to Stata 2/28/2001 is just the result of two divisions and will not be interpreted as you wish.

. di 2/28/2001
.0000357

Now 1 January 1960 is 0 as a daily date, so all dates in your data that are 2 January 1960 or later will be greater than this number and none will be equal and none will be less, as you found.

[tig som]

Thank you very much, Andrew Musau and Nick Cox!!