nor does it support British Summer Time (BST).

It's missing lots of timezones because it is continental-US focused for whatever reason.

So, do we try to add all known abbreviations for timezones? Or do we change TZ to be something like [A-Z]{3,4} to match any 3-4 character capitalized text?

Here is a pretty comprehensive list of timezones:
https://www.timeanddate.com/time/zones/

+1 to the [A-Z]{1,5} pattern (because some are only one letter (e.g. military G), and some are up to 5 letters (e.g. IRKST for Irkutsk Summer Time)

I've made a pull request with this change: #235 but I'm not 100% sure it's the right thing to do...
Pro: it would now actually match all timezone abbreviations which would more accurately match what one would assume it should do
Con: it is now one to five upper case letters from A to Z... that includes a lot more than actual timezones...

@robin13
I guess we don't want INFO and DEBUG matched though.

This is a list of abbrev (scraped from timeanddate.com page above), INFO DEBUG ALERT WARN should not be matched

ACDT ACST ACT ACWST ADT AEDT AEST AFT AKDT AKST ALMT AMST AMT ANAST ANAT AQTT ART AST AWDT AWST AZOST AZOT AZST AZT BNT BOT BRST BRT BST BTT CAST CAT CCT CDT CEST CET CHADT CHAST CHOST CHOT CHUT CIDST CIST CKT CLST CLT COT CST CVT CXT DAVT DDUT EASST EAST EAT ECT EDT EEST EET EGST EGT EST FET FJST FJT FKST FKT FNT GALT GAMT GET GFT GILT GMT GST GYT HDT HKT HOVST HOVT HST ICT IDT IOT IRDT IRKST IRKT IRST IST JST KGT KOST KRAST KRAT KST KUYT LHDT LHST LINT MAGST MAGT MART MAWT MDT MHT MMT MSD MSK MST MUT MVT MYT NCT NDT NFT NOVST NOVT NPT NRT NST NUT NZDT NZST OMSST OMST ORAT PDT PET PETST PETT PGT PHOT PHT PKT PMDT PMST PONT PST PWT PYST PYT QYZT RET ROTT SAKT SAMT SAST SBT SCT SGT SRET SRT SST SYOT TAHT TFT TJT TKT TLT TMT TOST TOT TRT TVT ULAST ULAT UYST UYT UZT VET VLAST VLAT VOST VUT WAKT WARST WAST WAT WEST WET WFT WGST WGT WIB WIT WITA WST WT YAKST YAKT YAPT YEKST YEKT INFO DEBUG ALERT WARN

here is one regex...

\b(?:((A[CEFKLMNRWZ]?[AMOW]?|AQT|B[NORT]?R?|C[ACEHIKLOSVX]?[AOU]?|DAV|DDU|E[ACEG]?S?|F[EJKN]|G[AEFIMY]?[LM]?|H[K]?|HOV|I[CORS]?[K]?|JAY|J|K[GOR]?|KRA|KUY|L[HIK]|LIG|LIN|M[AEHMPUVY]?[GRW]?|N[CFPRUZ]?|NOV|OMS|ORA|P[EGHKMOWY]?[ONT]?|QYZ|RE|ROT|S[ABCGR]?[EKM]?|SYO|T[FJKLMORV]|TAH|TRU|U[CUYZ]?|ULA|V[EU]|VLA|VOL|VOS|W[AEFG]?|WAK|WAR|WET|WI|Y[AE][KP])(D?ST|D?T|T)|BRA|MEZ|MSD|MSK|WIB|WITA|UTC|ZULU|Z))\b

@robin13 WDYT?

I think we should create a MIL_TZ with [A-IK-Z] no J at all.

Nice work @guyboertje ! :)
I'm not sure about excluding military - they are actually used timezones... many Grok patterns can match on things which they are not intended for, but it's all about sequence and context. If your pattern is:

%{DATE_EU} %{TIME} %{TZ}

This would match nicely against 01.01.2018 12:00:00 G but it would not match against This is Great even though the string contains two potential single-letter timezones (T and G).

@robin13

I guess the final Z in that regex can be replaced with [A-IK-Z]