LabVIEW
How can I get a UTC timestamp?
Solved!
The "Get Date Time in Seconds" VI does not have a UTC option. I have an application that needs the UTC as a timestamp.
I could use the "Seconds to Date/Time" with the UTC input as True. The output of this gives me the UTC in a cluster. This is shown in the upper branch of the VI above. The output shows me the correct UTC.
To try and get around this, I used the "Seconds to Date/Time" with the UTC input as True, followed by a "Date/Time to Seconds" with the UTC input as False. This is shown in the bottom branch of the VI above. I expected the output of this to give me the UTC as a time stamp.
Here's the problem: for some reason, the Time stamp output is 1 hour ahead of the real UTC. Can anybody explain this? Any workaround?
Be careful; the code is doing something a little different what you think it is doing, and won't work correctly during daylight savings time transitions.
LabView timestamps (the underlying numeric data on the wire) are always in UTC. The conversion to localtime is done only when a timestamp is displayed in a timestamp control/indicator/constant or written out to a string. You can verify this by casting a timestamp to a double.
If the problem is "how do I am make the timestamp control display UTC?", that is best done via a custom format: use the %^<>T container in "display properties":
Non-obviously, this setting applies to constants also. If you set a constant using the default (localtime) view, then quit LV, change your timezone, and restart, the constant will appear to change. It hasn't really changed, in that the underlying number-of-seconds-since-1904 is still the same, it's just displaying it differently. (It would be nice if LabView gave us some kind of visual to indicate whether a control/indicator/timestamp is showing local or universal time.)
The problem with the code above is that it says "display a localtime that has the same numeric digits as the current UTC time". It modifies the time on the wire to represent a different time than "now"; it's "now" offset from "now" by your TZ offset. The second is doing it this way will work "most of the time" but it has weird quirks (ask me how I know!) during daylight savings time transitions. For example, the UTC time string "2014-03-09 02:30" is a perfectly valid UTC time, but does not exist in the Eastern Time Zone: passing this time with UTC = F to Date/Time to Seconds will return the UTC time associated with "2014-03-09 03:30", and that's what will be displayed on the timestamp.
