write function to convert to date or delete

I have a dataframe that looks like the below. I want to convert the
Captured.Time  field to a date object. but some of the entries are not
properly formated and I get a message saying
Error in as.POSIXlt.character(as.character(x), ...) :
  character string is not in a standard unambiguous format.

So I want to write a function that will convert or delete. I could not
figure out how to do that and so I tried to write a function that would
convert or replace with text like noDateHere but that did not work either.
Can anyone tell me how to accomplish this?
Here is what I tried:

convertOrOmit=function(dt){tryCatch(as.POSIXct(dt),error=print("noDateHere"))}

  X       Captured.Time Latitude Longitude Value Unit Location.Name
1 12696963 2012-08-07 11:00:51 39.16094  140.4883    45  cpm
2  2056198 2013-11-10 03:14:19 32.84428 -117.2240    47  cpm
3   727957 2014-01-28 04:47:54 35.80605  139.3789    28  cpm
4  2864220 2013-10-22 19:41:53 35.07816 -106.6123    50  cpm
5  5787688 2013-06-13 04:13:57 35.83174  136.2027    35  cpm
6  6191345 2013-05-28 06:48:34 34.78944  137.9496    32  cpm
  Device.ID                           MD5Sum Height Surface Radiation
1        NA b0465019b46289b82450c39ce1397b98     NA                NA
2        NA 8fa14a1227d23e6cf286785e8843cc39     NA                NA
3        NA c72cd7f9cedd59cf6e6892049dfbf9a0     NA                NA
4        NA aca82e39ff9098e45eea04f661f68dc7     NA                NA
5        NA cc9394e6dceb91f0e0de97cc2db57e19     NA                NA
6        NA f18d194a41e1448c7776dbeba8b351af     NA                NA
               Uploaded.Time Loader.ID
1  2012-08-13 19:16:10.18555     10832
2 2013-12-05 01:47:24.154971     13958
3 2014-01-29 22:55:39.138043     14451
4 2013-10-26 13:50:17.629869     13743
5 2013-06-16 16:17:21.148239     12930
6 2013-06-04 23:31:55.455323     12841

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